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viking
02-25-2008, 03:02 PM
I had a crash today with my Sheldon lathe and it ate the teeth off the gear in the reverser/lead screw drive assy. The gear was phenolic so it sheared the teeth without further damage. I called National Acme ( dealer for Sheldon parts) and was told the gear was not in stock but was available. I asked how much and was told $595 and it would take 4 to 6 weeks. When I regained conciuosness I said thank you , have a nice day. He was insulted that I thought that was a rediculous price.

I ordered an involute gear cutter from MSC and will make my own. I now need to decide on a material that will have properties similar to the original phenolic. If this happens again I want the gear to shear rather than tear up the whole gear train. If using a plastic material of some kind I will press a brass bushing in the center. The gear is engaged any time the lead screw is turning so wear is a factor.

What suggestions to you all have?

tattoomike68
02-25-2008, 03:18 PM
NYLATRON
http://www.gcip.co.uk/EP/materials/nylatron.htm

We have used it for gears that run the governers for hydroelectic turbines at the dams here in the northwest. its good stuff.

TGTool
02-25-2008, 04:04 PM
That was probably a filled phenolic which is often used like that. I note that McMaster Carr has it, maybe listed as or under Garolite. Micarta is another brand name.

rantbot
02-25-2008, 04:08 PM
You could make it out of phenolic.

Grade XX Garolite is a phenolic which might be pretty close to what your gear was made from. McMaster-Carr sells it in big sheets, or in nice small gear-sized pieces.
__________
Oops, beat to it.

Peter N
02-25-2008, 04:12 PM
Probably made from tufnol, this was (later) standard practice on tumbler reverse gears on Myfords.
Info here, look under>techical>tufnol gears : http://www.tufnol.com/tufnol/default.asp

Incidentally Sir John makes and sells these as replacement parts in batches.

Peter

John Stevenson
02-25-2008, 04:42 PM
Tufnol which is just a trade name is used for these sacrificial gears just as Peter has said.

You can also buy a steel gear and bush it with another steel bush for a rotating fit and drill a 3/32" hole part way in both and fit a bit of brazing rod in.

Then if you get a crash it just shears the brass pin, new pin and you are back running with virtually no down time.

.

BobWarfield
02-25-2008, 05:19 PM
Tufnol which is just a trade name is used for these sacrificial gears just as Peter has said.

You can also buy a steel gear and bush it with another steel bush for a rotating fit and drill a 3/32" hole part way in both and fit a bit of brazing rod in.

Then if you get a crash it just shears the brass pin, new pin and you are back running with virtually no down time.

.

Brilliant!

That's what I'd be doing were it my baby.

Best,

BW

viking
02-25-2008, 07:48 PM
Yeah the shear pin idea would be a good one except its not driven from it's center it's just basicaly an idler gear between two driven gears and simply rides on a bushing on a smooth shaft.

GKman
02-25-2008, 08:34 PM
Tufnol which is just a trade name is used for these sacrificial gears just as Peter has said.

You can also buy a steel gear and bush it with another steel bush for a rotating fit and drill a 3/32" hole part way in both and fit a bit of brazing rod in.

Then if you get a crash it just shears the brass pin, new pin and you are back running with virtually no down time.

.

I'm a little dense. Is the result like a 1/2 round keyway in the gear and bush with the 3/32" rod as a key?

Regards to Robin, Maid Marion, and the rest of the band in the forest.

TGTool
02-25-2008, 11:57 PM
I'm a little dense. Is the result like a 1/2 round keyway in the gear and bush with the 3/32" rod as a key?

Regards to Robin, Maid Marion, and the rest of the band in the forest.

No, I believe the pin is transverse like a roll pin to hold the bushing and the gear together rather than a shaft key configuration.

viking
02-26-2008, 10:07 AM
Thanks for all the suggestions. I'm going to try nylatron as tattoomike68 sugested as I can get it locally and it machines well and is very reasonable in cost.

lazlo
02-26-2008, 10:15 AM
Nylatron is great stuff, but the reason phenolic/Tufnol/Garolite/Micarta is used in lathe headstock gears is because it's one of the few materials that damps vibration (in the gear train) better than grey cast iron.

Evan
02-26-2008, 11:05 AM
I would want to know the comparative impact resistance of the original gear compared to Nylatron or acetal. Acetal and Nylatron both have impact resistance equal to or greater than a steel gear of the same dimensions. It is likely that the "fuse" properties of the phenolic gear will be lost by using either plastic.

John Stevenson
02-26-2008, 11:28 AM
But does the plastic have a shear strength greater than steel because that's what happens when these go.
One stops, one carries on and it's a guillotine effect.

.?

.

lazlo
02-26-2008, 11:58 AM
I would want to know the comparative impact resistance of the original gear compared to Nylatron or acetal. Acetal and Nylatron both have impact resistance equal to or greater than a steel gear of the same dimensions.

The impact strength may be better, but I bet the wear resistance (limiting PV and K ratings) is lower on Delrin or Nylatron than phenolic, due to the glass-filled content.

Heck, even on the Nylatron family, there's a 20x difference in wear resistance between the unfilled Nylon 6 (normal Nylon), Nylatron GSM (Molybdenum filled), and Nylatron NSM (some secret formula that Quadrant hasn't revealed).

Delrin K wear resistance (K rating) is far lower than Nylatron: 211 In-min/
Ft-Lbs-Hr, versus 83 for Nylatron GSM, and 8 for Nylatron NSM (which is equal the wear resistance of bronze, by the way...).

That means that a Delrin gear would wear about 3 times as fast as a Nylatron GSM gear, under the same pressure/velocity conditions.

Nylatron was specifically formulated for high wear gears, bushings, ...

QSIMDO
02-26-2008, 12:05 PM
Geez, I was upset when I found a phenolic gear in my Grizzly.

Learn something every day.

Scishopguy
02-26-2008, 03:37 PM
Viking...I had to replace one of those on the sheldon we had at work about ten years ago. I had to track down the source for parts, that had been sold when Sheldon got out of that part of the biz. I paid $50.00 each for them. I got an extra since we had some temporary help working in the shop. I am astounded that they are trying to get that much for a straight cut, linen phenolic gear with a bronze bushing in the center. You should be able to find some sort of replacement but if not, it should not be hard to make on the mill.

hang in there, we're all pulling for ya!

Evan
02-26-2008, 08:28 PM
But does the plastic have a shear strength greater than steel because that's what happens when these go.
One stops, one carries on and it's a guillotine effect.

Who cares? The question is how does it compare to the phenolic gear.

lazlo
02-26-2008, 09:12 PM
Who cares?

I didn't understand John's comment either. Seems like the teeth on the sacrificial gear would just shear off (which is what you want), whether it's thermoplastic or phenolic.

oldtiffie
02-26-2008, 10:17 PM
Tufnol - or its equivalent - was used for many years on a locally produced (OZ), USA-designed GM 6-cylinder OHV engine (no OH cam-shaft). The Tufnol gear was on the front of the crank-shaft on the valve timing gear train and operated in an oil-rich environment in a sealed case.

It was quite/very successful and had a quite good operating life providing the engine was not over-revved or "thrashed". It did not take kindly to back-fires either.

It almost invariably failed when wear caused "crowning" of the gears at which point the Tufnol gear teeth broke or sheared off and the remaining steel gears stayed intact and in most cases were almost as new.

Tufnol would be my material of choice if I were to make a gear for the gear train on my lathe. I would hesitate to use it in the geared head-stock though, only because of caution and pending qualified info as to its suitability.

rantbot
02-26-2008, 10:41 PM
The suggestion on page 1 to replace a phenolic gear with another phenolic gear was not made frivolously.

Lacking further information, it's always wise to assume that the guys who designed a machine had at least a bit of an idea of what they were doing. If they thought it should be phenolic then, then maybe it should be phenolic now.

If the material is unavailable, that's another matter. But that's hardly the case in this instance.

Evan
02-27-2008, 02:31 AM
That is my advise as well. Plastics such as Nylatron and Delrin are stronger than most people think. Acetal (or Delrin) are commonly used in washing machine transmissions and the cams in a Briggs and Stratton are acetal. I use both of these plastics and machine them regularly. It is surprising what happens when a chip becomes jammed in the bottom of a large hole while drilling on the lathe. You almost can't drill it out. Nylatron is famous for grabbing a drill bit so tight that it can't be removed. It also is very unstable dimensionally in the presence of high humidity as it can absorb up to 7 or 8 percent water. Acetal however absorbs almost no water and when run in an oil rich environment exhibits almost zero wear. Both are very difficult to shatter or break. They tend to give and deform instead. Fabric reenforced Phenolic OTOH, tends to tear and shred.

John Stevenson
02-27-2008, 03:36 AM
I didn't understand John's comment either. Seems like the teeth on the sacrificial gear would just shear off (which is what you want), whether it's thermoplastic or phenolic.

Ok I'll try to make it clearer.

Put the gear train under load and the teeth will shear off as needed but at what load.

You don't want the teeth shearing off just because you put a couple on extra thou on.

Imagine it as a fuse. Phenolic is rated at 15 amp and is correct.
If the plastic one is 5 or 10 amp then you are going to have unnessary problems, I might have the terms wrong but shear stress seems to fit the bill.

.

J Tiers
02-27-2008, 08:11 AM
I ask because the phenolic /micarta gears were RARELY used for their "breakability". Instead, there were other reasons for using them.

Particularly I am surprised by the use of such a "gear-fuse" concept by a good manufacturer like Sheldon.

While I KNOW that Atlas used breakable zamak parts as fuses (leadscrew bearing), a "higher level" manufacturer like Sheldon would surely have used a shear pin or even a clutch for such a purpose.

For one thing, the "release point" would be far better known with a pin or clutch. It could be "designed" to break when needed, and hold below that. Plus, industrial managers have a very dim view of specialty parts which are "designed to break", and require disassembly for replacement. Pins are OK, they can be made locally, and normally are easy to replace, saving the machine from downtime due to breakage of expensive parts.

It is very likely that any shop would have replaced that busted gear with steel and got on with the job.

INSTEAD, I offer you THE PRIMARY REASON FOR USING PHENOLIC, originated by GE......

NOISE, and wear resistance.

GE found that materials of that type could OUTLAST steel in some applications. They actually used CLOTH gears, layers of cloth sandwiched between steel end pieces.

They found that such gears could last several times as long, and had far less noise output, than steel or cast iron gears. The phenolic was a later development along the same lines.

So I SUGGEST TO YOU THAT THE GEAR IS NOT A "FUSE". I suggest that it was used purely and simply as a noise reducer.

I understand that the bull gear on a South bend shaper is also phenolic. That would be a very poor part to make as a "fuse", it would require stripping the machine to replace.

lazlo
02-27-2008, 10:51 AM
Imagine it as a fuse. Phenolic is rated at 15 amp and is correct.
If the plastic one is 5 or 10 amp then you are going to have unnessary problems,

Ah, got it John -- good point.

On a someone unrelated note -- has anyone ever found Nylon/Delrin/Nylatron or Phenolic gear blanks with metal hubs?
I once saw a Nylatron gear blank cast around a steel insert described in some manufacturer's catalog, but I forgot to bookmark it, and I've never seen them for sale...

JCHannum
02-27-2008, 12:17 PM
[QUOTE=J Tiers]Particularly I am surprised by the use of such a "gear-fuse" concept by a good manufacturer like Sheldon.

So I SUGGEST TO YOU THAT THE GEAR IS NOT A "FUSE". I suggest that it was used purely and simply as a noise reducer.[QUOTE]

Sheldon used three phenolic gears in the drivetrain, the other gears were brass, steel and cast iron. That particular gear is the first gear, and the smallest, least expensive gear in the train. It is the gear that will fail in the event of a catastrophic jam. I have little reason to doubt that it is considered sacrificial to protect the more expensive portions of the gear train.

A taper pin is incorporated in the final drive of the QC gearbox, but the phenolic gear will fail before that. A slip clutch is a non-starter in this case, the apron feed has a clutch that will slip, but when threading, a slip clutch will lead to other problems, hence the shear gear.

Evan
02-27-2008, 02:46 PM
I once saw a Nylatron gear blank cast around a steel insert described in some manufacturer's catalog, but I forgot to bookmark it, and I've never seen them for sale...

They are as common as dirt in office machines. I have a box full of various types. I suspect you won't find them retail because there are so many possible options, nylon, Delrin gears, aluminum, brass or steel hub, cast in bearings, press in hubs and bearings, one way clutches, ball, needle or oil impregnated, sintered, turned or cast hubs etc.

In office machine use the metal gears usually fail long before the plastic gears. Surprisingly, the main problem with the plastic gears with plastic hubs (no bearing) is that they wear out the steel shafts long before the gear fails. Very rarely have I ever seen a plastic gear missing teeth. They don't break off regardless of the type of drive system fault.

viking
02-27-2008, 03:10 PM
Tons of great information . A phenolic replacement is probably good advice for many of the reasons you have all provided. McMaster Carr sells Garolite but in my catalog(an older hand-me-down version) you either have to buy 4 feet of 2" round or a 12" X 12" square of 1" flat. I will continue to look for a source of 2" round that I can buy by the foot.

In the meantime I am going to make a gear in Nylatrol for the following reasons:

1. I have never cut a gear like this before and It will be a good exercise and learning experience. I can apply what I learn here to make a phenolic gear.

2. I will have very little invested in it as I already have the Nylatron and a brass bushing.

3. When completed, the Nylatron gear will give me the ability to use the powerfeed to construct the Garolite gear.

Thanks again for all the help!!!

viking
02-27-2008, 03:24 PM
Just checked McMaster Carr website and you can buy 2" round Garite LE for $54 for a 1 foot peice.

John Stevenson
02-27-2008, 03:35 PM
DON'T USE ROUND FOR GEARS.

The round material is wound in a continuous strip, rather like a roll of tape.

The sheet material is laid up like plywood.

Using the round puts the bottom of the teeth along a line of weakest construction.

Use sheet and either cut a blank out with a hole saw or band saw it and chop the corners off and final machine on a spigot or arbor.

.

JCHannum
02-27-2008, 03:44 PM
You don't want round stock, get a 3/4" X 4" X 6" flat for about 1/3 the cost of the round stock. It will make a better gear due to the orientation of the grain.

John beat me to the draw, great minds and all that.

Evan
02-27-2008, 07:11 PM
Here is an example of change gears made from acetal and Nylatron. I made both of these by milling from flat with my CNC mill. They are a 37/47 metric transposing compound. I used a 1/16 carbide endmill that I modified by grinding the shank so it could cut a gear full width. The involute shape came out pretty well, certainly good enough for change gears.

http://vts.bc.ca/pics3/gears1.jpg

http://vts.bc.ca/pics3/gears2.jpg

lazlo
02-27-2008, 09:43 PM
Evan, the Nylatron GSM that I use is graphite (molybdenum disulfide) colored. Is your navy blue gear unfilled Nylatron, and the black gear colored acetal?

Evan
02-27-2008, 10:04 PM
The blue-gray is moly filled. Graphite colored nylon is normally graphite filled, a different product entirely and sometimes called ballistic nylon.


The left is Nylatron, the center is graphite filled nylon and the right is natural nylon.

http://vts.bc.ca/pics/plastic1.jpg

The black gear is acetal.

[added]

Molybdenum disulfide isn't black although it may appear so in some formulations. It's blue-gray. I used to service equipment at a molybdenum mine and the entire surrounding landscape was blue-gray from drifting dust from the crusher.

lazlo
02-27-2008, 10:18 PM
The blue-gray is moly filled. Graphite colored nylon is normally graphite filled, a different product entirely and sometimes called ballistic nylon.

Unfortunately not Evan. The blue Nylatron is Nylatron 901, also called "Nylatron Blue" which is the "common" unfilled Nylatron.
Nylatron GS/GSM is graphite colored. Nylatron NSM is tan.

This is a chunk of Nylatron GSM that I bought last week. I've got a bunch of it, and I have the certs:

http://i164.photobucket.com/albums/u15/rtgeorge_album/NylatronGSM.jpg

lazlo
02-27-2008, 10:25 PM
The blue Nylatron is Nylatron 901, also called "Nylatron Blue" which is the "common" unfilled Nylatron.
Nylatron GS/GSM is graphite colored. Nylatron NSM is tan.

This is from Quadrant's (the manufacturer) datasheet:

http://www.portplastics.com/download/pdf/plastics/BearingAndWear/bearingAndWear11.pdf

Nylatron Sheet & Rod

MC 901 Nylon. Heat stabilized Nylon. It is blue in color.

Nylatron GSM Nylon. Contains finely divided particles of molybdenum disulphide (MoS2) to enhance its load bearing capabilities while maintaining the impact resistance inherent to Nylon. It's the most commonly used grade for gears, sheaves, sprockets and custom parts. It is gray-black in color.

Evan
02-27-2008, 10:32 PM
Blue Nylatron is BLUE, not gray-blue. Nylatron GS, which is what I have, (NOT GSM) is molybdenum disulfide filled and is blue/gray in color. "Nylatron GSM Blue" is BLUE and is both moly and oil filled. Nylon MC 901 is also BLUE, not gray-blue.

lazlo
02-27-2008, 10:39 PM
Nylatron GS, which is what I have, (NOT GSM) is molybdenum disulfide filled and is blue/gray in color

Nylatron GS and Nylatron GSM are the same thing. GS is extruded, and GSM is cast. Both are grey:

Nylatron® GSM - Cast MOS2-Filled Nylon 6

Nylatron GSM cast nylon contains finely divided particles of molybdenum disulfide to enhance its load bearing capabilities while maintaining the impact resistance inherent to nylon. It is the most commonly used grade for gears, sheaves, sprockets and custom parts. It is gray-black in color.

Nylatron® GS – Extruded Nylon 6/6, MoS2 filled

Nylatron GS Nylon is a nylon and molybdenum disulphide (MoS2) composition designed to improve the mechanical, thermal and bearing properties of type 6/6 nylon while maintaining its basic electrical and chemical characteristics. Nylatron GS offers greater wear resistance, lower surface friction, higher strength and greater rigidity than unfilled 6/6 with improved dimensional stability. It is gray-black in color.



"Nylatron GSM Blue" is BLUE and is both moly and oil filled.

Nylatron GSM Blue is yet another Nylatron variant: it's oil filled Nylatron GSM. You'd know if you had it: it weeps oil all over the place, especially when you cut it.

Evan
02-27-2008, 10:41 PM
Nylon MC901
http://www.professionalplastics.com/professionalplastics/content/mc901-thumb.jpeg

Nylatron GSM Blue

http://www.professionalplastics.com/professionalplastics/content/100444-1-thumb.jpg

Nylatron GS

http://www.emcoplastics.com/images/sce/nylatron.GIF


Perhaps your monitor needs adjustment? Your stick of GSM looks the same color as mine.

lazlo
02-27-2008, 10:43 PM
Evan, here's a picture of the Nylatron 901 (unfilled Nylon) that you have:

Nylatron Nylon 901
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200100143947

http://i98.photobucket.com/albums/l269/ericphoto_2006/nylon%20photos/Ap2r2007DCFN0011.jpg

lazlo
02-27-2008, 10:49 PM
Nylatron GS

http://www.emcoplastics.com/images/sce/nylatron.GIF

You linked to the wrong picture Evan. This is the correct URL from your web page (professionalplastics.com):

Nylatron® GS Nylon - Extruded
http://www.professionalplastics.com/cgi-bin/pp.pl?pgm=co_disp&func=displ&strfnbr=3&prrfnbr=100445&sesent=0,0&search_id=124523
http://www.professionalplastics.com/professionalplastics/content/nylatrongsnylon-full.jpg

Evan
02-27-2008, 10:50 PM
Have you had your color vision checked lately?

Here is a selection from your pic beside mine with absolutely no adjustments made.

http://vts.bc.ca/pics3/nylatrongs.jpg

Evan
02-27-2008, 10:53 PM
You linked to the wrong picture Evan.
No, I didn't. I linked to another site because the image you posted is a grey scale palette. It isn't color.
[edit] It has 249 shades of gray.

lazlo
02-27-2008, 10:55 PM
Have you had your color vision checked lately?]

Ahem. I have 20/15 vision, and I'm not colorblind Evan.


Perhaps your monitor needs adjustment? Your stick of GSM looks the same color as mine.

Now this is Nylatron GS/GSM -- (the same stuff I posted). It's graphite colored (Quadrant, the manufacturer, calls it "gray-black"):
http://vts.bc.ca/pics3/nylatrongs.jpg

But that's not what you posted originally! This Navy Blue nylon looks like common Nylon 901:

http://vts.bc.ca/pics3/gears1.jpg

Evan
02-27-2008, 11:00 PM
But it isn't even close to navy blue. It's exactly the same stock as in the other picture. And, nylon MC901 isn't close to that color either.

lazlo
02-27-2008, 11:02 PM
But it isn't even close to navy blue. It's exactly the same stock as in the other picture. And, nylon MC901 isn't close to that color either.

Nylatron Nylon 901
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200100143947

http://i98.photobucket.com/albums/l269/ericphoto_2006/nylon%20photos/Ap2r2007DCFN0011.jpg

Shrug. Maybe it's your lighting, but Nylatron GS or GSM (same stuff) is pure grey colored, like the roll you have on edge on post 42.

On a side-note, the vibrant blue Nylatron GSM Blue (the oil-filled Nylatron) is neat stuff too. It's also sold as "Nyoil" (not to be confused with the lubricant of the same name :) ).

I'm dying to get a chunk of Nylatron NSM, but it's $60/lb (!) It's got the wear resistance of bearing bronze (about 10 times the wear resistance of Nylatron GSM, and 20 times Delrin) but a coefficient of friction of .078 (less than half the friction of the other Nylatrons).

In any event, I think we bored everyone else with our plastic fetish :D

Evan
02-27-2008, 11:06 PM
Yeah, but that 901 sample is much bluer than my and your samples.


Maybe it's your lighting,

I take nearly all my photos in my shop which is all daylight white CFL bulbs and also use white LED lighting for highlighting. In the shot of the gears I did use white LED fill light.

bhjones
05-20-2008, 05:24 PM
Viking, what was the outcome of your Sheldon repair?

I own a Sheldon EXL and I just sheared some teeth off the same 32 tooth phenolic gear.

JCHannum
05-20-2008, 06:05 PM
I don't know about the success of Viking's repair, but I have made replacement gears for my Sheldon using Micarta sheet. It is no different than any other gear cutting procedure. It is 16DP, 14-1/2 degree PA.

The brass sleeve presses out of the original gear and can be reused, or a knurled replacement made if the bore is sloppy.

J. R. Williams
05-20-2008, 06:50 PM
My Clausing-Colchester lathe has one of the Linen filled Phenolic resin gears in the train and in addition a soft steel shear pin on the input and output shaft of the gear box and one on the lead screw. The lathe is 26 years old and has around 3000 operating hours.

JRW

wierdscience
05-20-2008, 11:34 PM
What John is refering to will work well on a driving or driven gear where the load is applied shaft bore through teeth.It won't work on an idler gear since the load is transmitted teeth to teeth.

Bes to replace the gear if it's an idler with any old steel change gear and then convert the driven gear on the leadscrew or change gear input to a shear key.This offers the best protection since the last gear in the gear train sees the highest torque at the lowest speed.

Edited for late post

bhjones
05-21-2008, 12:06 AM
Well I'm happy to say this gear, if it was intended to be a failure point or not, did that job very well. I on the other hand did a poor job.

I was getting ready to do some threading on a short bit of stock between centers and failed to double check the direction of carriage travel. I got everything setup and engaged the halfnuts then the carriage moved 1/4 inch or so right into the tailstock and the gear teeth started to shear off. I was cutting 8tpi so there was no time to release the halfnuts.

Since it saved my bacon today I'll be using the same material to replace the gear.

Here are a couple of photos of the carnage (warning, unedited for size. their big):

http://www.5foot2.com/images/img_0130.jpg
http://www.5foot2.com/images/img_0135.jpg

Astronowanabe
05-21-2008, 01:27 AM
I too am awaiting a replacement tumbler gear for my 10" logan.

Saturday night I had finished making a boring bar and holder for one of the
those snail looking thread cutters and decided to try making a 1" 8tpi arbor
(against a shoulder) out of a hi grade bolt head. seemed if I could do that I could be less apprehensive about the rest of learning to single point threads ...

over all it worked like a champ,
true I did learn the depth of the cut you can take falls off as the thread gets deeper. but the flat belts spun off as they are suppose to and after backing off a bit things proceeded smoothly.

that is until I was making a free pass at the end. then with the feed engaged but the cutter still several inches from the work the lathe stopped belts spun off. it was just one of the gears lost 8 of 30 teeth. the gear (all the gears) are cast iron.

what has me perplexed is why all the teeth would have broken off the same gear? there was one stretch of three teeth in a row the rest were spread out pretty evenly around the rest of the gear.

the broken tooth fragments made their way around gears on either side
of the gear they came off of but I have the whole gear train disassembled and find no additional damage beyond this one gear.

in terms of replacing it I have already ordered a tumbler assembly off of ebay
which should give m a replacement and a spare.
I do like the idea of putting a shearpin in the gear that drives the lead screw
what would the fit between the inner and outer portions of the gear be?
a light press or loose fit and what sort of shear strength would the pin need?

rantbot
05-21-2008, 03:53 AM
In principle the Logan leadscrew drivetrain already has a mechanism to disengage the gears in case of overload. The gear change bracket (Logan # LA-227) (http://bridgeport.askmisterscience.com/la-10.gif) pivots around, and clamps on to, the left hand leadscrew bracket. When the carriage or cross feed strikes an immovable object, the gear change bracket can pivot downward sufficiently to allow the gear train to disengage. Or at least it can if the bracket's clamp bolt isn't socked down deadly tight. It's not really a "positive" system in that there's no way to set it "just right," except by guess and by gosh.

J Tiers
05-21-2008, 08:30 AM
The whole concept of a breakable gear seems deadly stupid, compared to a shear pin, for instance.... or a clutch, maybe. Logan and others made "safety gears" with a clutch built-in.

Sure it can save your bacon, but at what cost?

A shear pin can be made to give at a very specific force, is cheap, and in the end is no more likely to be replaced with the wrong thing than a gear.

After all, while a nail may be dropped in the pin hole, at least then you are still running production, and presumably being more careful for now.

An expensive breakable gear is far more likely to be replaced permanently with something more solid, and meanwhile you are "down" and not producing parts.....

The shear-pin setup need not be any more expensive than an exotic 2 piece gear.

So, the breakable gear seems like the most idiotic and brain-dead solution imaginable...................... A number of far better solutions exist. And did "back then".

lazlo
05-21-2008, 11:11 AM
The whole concept of a breakable gear seems deadly stupid, compared to a shear pin, for instance.... or a clutch, maybe. Logan and others made "safety gears" with a clutch built-in.

If you read through the first half of the thread, I pointed out that, according to Tony's lathes.co.uk page, phenolic gears are used in even high-end lathes because linen phenolic is one of the very few materials that damps vibration better than grey flake cast iron.

The phenolic gears are therefore not cost engineered or cheap, and they're not intended to function as a shear pin.

Jerry: I know you're a fan of Robert Smith's "Advanced Machine Work." There's a section on gear-cutting titled "Cutting Silent Gears." It's about cutting phenolic gears.

One of these days I want to add a linen phenolic gear to the quadrant on my Clausing. You only need one gear to break the chain of vibration transmission to the leadscrew...

bhjones
05-21-2008, 08:45 PM
Sure it can save your bacon, but at what cost?
.

About the cost of a phenolic gear, or more likely in my case the cost of the gear cutter. Since I do this machining stuff for fun, it seem like it'll be fun to finally put my dividing head to work and make a gear with it.



...at least then you are still running production...

...and meanwhile you are "down" and not producing parts.....


Production? My shop is all about enjoyment, not production. Being delayed in the work I was doing is no big deal, and being side tracked into a repair should prove to be enjoyable.

bhjones
05-21-2008, 08:52 PM
The tumbler on my Sheldon had the phenolic gear running against a brass gear. There is a huge difference between the gear noise dependent on which one is riding on the spindle gear.

There are two other phenolic gears used between the spindle and gear box but none of them ride against each other.

Can a phenolic gear run against another phenolic gear?


If you read through the first half of the thread, I pointed out that, according to Tony's lathes.co.uk page, phenolic gears are used in even high-end lathes because linen phenolic is one of the very few materials that damps vibration better than grey flake cast iron.

The phenolic gears are therefore not cost engineered or cheap, and they're not intended to function as a shear pin.

Jerry: I know you're a fan of Robert Smith's "Advanced Machine Work." There's a section on gear-cutting titled "Cutting Silent Gears." It's about cutting phenolic gears.

One of these days I want to add a linen phenolic gear to the quadrant on my Clausing. You only need one gear to break the chain of vibration transmission to the leadscrew...

J Tiers
05-22-2008, 01:43 AM
Laslo, your post is ironic.......

I alluded to the OTHER benefits of a linen-phenolic gear early on in this thread, suggesting that the "fuse" aspect wasn't the likely reason for use..... mentioning the GE cloth gears which were the origin of the Micarta type.

I got roundly dumped-on for suggesting that, too......... But anyhow I AM quite aware of the more appropriate reasons for using that type gear.....


I ask because the phenolic /micarta gears were RARELY used for their "breakability". Instead, there were other reasons for using them.

Particularly I am surprised by the use of such a "gear-fuse" concept by a good manufacturer like Sheldon............

..............INSTEAD, I offer you THE PRIMARY REASON FOR USING PHENOLIC, originated by GE......

NOISE, and wear resistance.

GE found that materials of that type could OUTLAST steel in some applications. They actually used CLOTH gears, layers of cloth sandwiched between steel end pieces.

They found that such gears could last several times as long, and had far less noise output, than steel or cast iron gears. The phenolic was a later development along the same lines.

So I SUGGEST TO YOU THAT THE GEAR IS NOT A "FUSE". I suggest that it was used purely and simply as a noise reducer.

I understand that the bull gear on a South bend shaper is also phenolic. That would be a very poor part to make as a "fuse", it would require stripping the machine to replace.

oldtiffie
05-22-2008, 06:32 AM
Unless my memory fails me, the method John Stevenson mentions - putting an axially aligned round pin on the junction between a wheel and its pinion - is a variation of what was called a "Scotch key" as it was a quick, cheap and effective key at minimal cost.

And it works very well. Brass is better than copper which work-hardens over time. Steel has a higher shear strength than brass and "mystery metal" is just about a "no-no". Brass is a pretty well known quantity.

I have lots of faith in flat-sheet "Tufnol" as despite its seeming expense it is easily cut on a band-saw, lathe and mill. It lasts very well in oil or abrasive environments (such as lathe change-wheels which are seldom in a closed box and rarely disengaged).

The GM arm in OZ made copies of their Chevrolet engines here with Tufnol gears on the crank-shaft OHV drive - and they lasted for years and many thousands of miles.

There is no real reason that "Tufnol" gears should not be used in a change-wheel gear-train - at all. They will work very well as a "fuse"/over-load device and will shear off their teeth if over-loaded - preferably on one of the larger gears for maximum torque = thrust = shear(ing).

I was taught many years ago to set/tighten the quadrant (aka "banjo") under the lead-screw gear to the least that would do the job (keep the gears in mesh under load) as the 20 degree thrust on the gears was sufficient to make the gears move out of mesh and so save the gears.

I still do it. I hit my chuck once when I was screw-cutting and the quadrant moved the gears out of mesh.

The discussion this far seems to have been only with regard to the potential failure of gears in the exposed gear train. The same can happen in a quick-change gear-box (QCGB). It can be extended to the power-feed mechanisms in the apron as well. The "fuse" or "quick disengagement" in the head-stock external change-gear train will be equally effective in all cases.

Me? I'd stick with "Tufnol" as I can knock up a gear pretty quickly.

One reservation I do have about "Tufnol" is running it on a shaft directly as the shaft may have more wear than if the bearing was bronze or steel. So I'd put the "Tufnol" gear on a steel/bronze/brass centre and fix them together with set-screws.

I am not saying that any method is right or wrong in absolute or relative terms at all. It is entirely up to each person to decide for himself in the circumstances in which he finds himself at the time.

Pete H
06-04-2008, 09:34 AM
I'm weighing in a bit late on this, but with interest anyway, because I have an Emco Maximat, which has phenolic gears in the headstock, as well as nylon AND nylon-with-steel-hub gears in the leadscrew train.

The composite nylon-and-steel gear engages the tumbler, which has two all-nylon gears. It's driven by a rollpin between its steel hub and the main pulley. The steel hub is "milled" like the edge of a coin, and the nylon rim is pressed on. Mine was cracked. I found a steel-hub gear at Quality Transmission Components in NY, but they wanted $103 (a 45 tooth, 47mm gear). However they also sell an all-nylon gear with a protruding hub, in the same size, for $20, so I got one, bored it out to the right size, and pressed it on the hub. It's sky-blue, and made in Hong Kong.

Thankfully, the headstock gears are in decent shape - just a bit "fuzzy" on one corner where some eejit (not THiS one) got careless. But someday they may need replacing.

I've been researching the properties of various materials, including Nylon 606 (not 66), which is what the blue gears from QTC were. 606, btw, is Teflon-filled.

I haven't been able to find shear strength for paper Micarta yet, but in comparison with Nylon 606 and filled and unfilled Delrin, glass-reinforced Micarta takes the prize for shear, flexural, and un-notched Izod (an impact-resistance test). Cotton Micarta comes out in the middle, except paper has higher flexural strength. And MUCH lower Izod.

This all leads to a question: I learned some years ago that the thing that causes damage in IC engines is grit getting stuck in the bearing metal - the softer stuff - and scoring the journals - the harder stuff - as they rotate.

SO.. will the glass in the glass-filled Micarta contribute to overall wear in the headstock?

Thanks.

lazlo
06-04-2008, 11:09 AM
I've been researching the properties of various materials, including Nylon 606 (not 66), which is what the blue gears from QTC were. 606, btw, is Teflon-filled.

I haven't been able to find shear strength for paper Micarta yet, but in comparison with Nylon 606 and filled and unfilled Delrin, glass-reinforced Micarta takes the prize for shear, flexural, and un-notched Izod (an impact-resistance test). Cotton Micarta comes out in the middle, except paper has higher flexural strength. And MUCH lower Izod.

For the headstock gears, what you really want to look at is material's PV rating (sustainable psi per fpm) and the K factor, which is wear in cubic feet per psi. Nylon has great impact resistance, but lousy PV and wear rating. Nylatron (gear and bearing nylon blended with various binders and lubricants) has much higher PV and wear ratings. If you flip through the bearing section of the MSC or McMaster catalogs, they have the PV and K ratings on all the materials, so you can compare/contrast the various options.

I've never heard of Nylon 606. I've seen/used Nylon 66 and 6/6, and there's Nyoil (Nylon impregnated with oil like an Oilite bearing). Do you have a link?

The abrasion question about the glass-filled phenolic is interesting. Gears are usually made from linen phenolic, and there are several grades of canvas and linen phenolics (all which use cotton fibers), so the abrasion probably varies a lot.

But anyone who's cut or drilled FR-4 (PCB phenolic) knows that it's incredibly abrasive. Maybe someone who's machined the fine linen phenolic like John, JC or Tiffie can weigh-in. According to Tony Griffith's page, many quality lathes have used phenolic gears to reduce gear-train noise, so I'd be really surprised if there's an inherent problem with abrasion.

Scishopguy
06-04-2008, 11:43 AM
The whole concept of a breakable gear seems deadly stupid, compared to a shear pin, for instance.... or a clutch, maybe. Logan and others made "safety gears" with a clutch built-in.

Sure it can save your bacon, but at what cost?

A shear pin can be made to give at a very specific force, is cheap, and in the end is no more likely to be replaced with the wrong thing than a gear.

After all, while a nail may be dropped in the pin hole, at least then you are still running production, and presumably being more careful for now.

An expensive breakable gear is far more likely to be replaced permanently with something more solid, and meanwhile you are "down" and not producing parts.....

The shear-pin setup need not be any more expensive than an exotic 2 piece gear.

So, the breakable gear seems like the most idiotic and brain-dead solution imaginable...................... A number of far better solutions exist. And did "back then".

I totally agree with you on this but the reason for the sacraficial gear is simply economics. When the phenolic gear on our sheldon got wrecked by a careless employee I ordered two replacements. We were under a deadline and I didn't want to have to wait on one again. They were $50 a pop so it wasn't a big deal for the department to order. Sheldon, or whoever took over their parts operation, wants to keep your money coming in. You will probably buy only one lathe but the spare parts thing can be an endless source of income.

Scishopguy
06-04-2008, 11:59 AM
[/QUOTE]
But anyone who's cut or drilled FR-4 (PCB phenolic) knows that it's incredibly abrasive. Maybe someone who's machined the fine linen phenolic like John, JC or Tiffie can weigh-in. According to Tony Griffith's page, many quality lathes have used phenolic gears to reduce gear-train noise, so I'd be really surprised if there's an inherent problem with abrasion.[/QUOTE]

I have not machined any of the FR-4 but have done a lot of the fine linen phenolic. A company I worked for made tray carriers for the airlines and they used the phenolic for strike plates for the latch mechanism. It was a 2"x4"x 3/8" piece with a step milled on each side, longwise, and a 1/4" slot milled for the latch bolt. The stuff machined beautifly (dry of course) but stunk like a skunk's butt in a forrest fire. You never forget that smell. ;)

bhjones
06-04-2008, 12:58 PM
I'm waiting on a stub arbor for my mill so I haven't cut the teeth on my replacement gear.

I'm really curious about wear issues when comparing glass to linen phenolic material. I'm using glass re-enforced phenolic for the gear blank. It'll be running against a bass and a steel gear. Should I stop where I am (I've inserted a bronze hub and turned the OD to size, but no teeth) and start over with linen?

Oldbrock
06-04-2008, 01:35 PM
Do not use brass for the bushing use bronze or sintered bronze. Brass makes a very poor bearing surface. Peter

Mad Machinist
06-04-2008, 05:04 PM
I drill through the joint between the gear and the bushing in two places so it is like a keyway.....but I fill with wheel weight metal.....nor as hard as the brass.....and will shear with about the same force as the phenolic.