PDA

View Full Version : Why are worm and rack trains not used often?



lowcountrycamo
12-10-2012, 10:20 PM
I was looking at a worm, wheel, and rack today that all had the same DP and tooth size (although worm and rack are really a trapazoids), and the worm mesh very well with the rack. I would think this would offer smooth accurate linear movement. I think this could be at least as simple as worm/wheel/screw train. Maybe the answer is obvious and I'm just numb.

Optics Curmudgeon
12-10-2012, 11:27 PM
What you're describing is an acme leadscrew and nut, only in this case the screw is short and the nut is long.

aostling
12-11-2012, 12:53 AM
I think this could be at least as simple as worm/wheel/screw train.

Driving a rack with a worm would be problematic. The rack would occupy the space where you would want to put the bearings supporting the worm.


http://i168.photobucket.com/albums/u183/aostling/Screenshot2012-12-10at105637PM.png

Sun God
12-11-2012, 01:23 AM
Driving a rack with a worm would be problematic. The rack would occupy the space where you would want to put the bearings supporting the worm.


http://i168.photobucket.com/albums/u183/aostling/Screenshot2012-12-10at105637PM.png

It seems the obvious way of doing such a thing to me would be to offset the axis of the worm from parallel to the rack's axis by the worm's helix angle - that way, the contact points on the worm would, from the rack's frame of reference, be translating along the worm axis, but parallel to the rack axis, if all of that makes sense? In essence, it then becomes a worm driving a helical gear, but the gear is unwrapped and projected along a flat plane. I'm not sure that makes it any more obvious either though. Maybe a diagram would help...

If you then need to increase the offset of the worm axis for futher clearance, increase the leade of the worm, and make it multi-start, to match the pitch of the rack.

philbur
12-11-2012, 04:24 AM
Not sure what you mean by the worm/wheel/screw train. Isn't it the same as a nut and a feed screw. The nut and screw has much more surface area for load carrying and for wear. It is also possibly easier to make. Single point tool on a lathe.

Phil:)

J Tiers
12-11-2012, 08:34 AM
It seems the obvious way of doing such a thing to me would be to offset the axis of the worm from parallel to the rack's axis by the worm's helix angle - that way, the contact points on the worm would, from the rack's frame of reference, be translating along the worm axis, but parallel to the rack axis, if all of that makes sense? In essence, it then becomes a worm driving a helical gear, but the gear is unwrapped and projected along a flat plane. I'm not sure that makes it any more obvious either though. Maybe a diagram would help...

If you then need to increase the offset of the worm axis for futher clearance, increase the leade of the worm, and make it multi-start, to match the pitch of the rack.

Nice outside the box thought.

Another possibility is that there is no law giving the maximum diameter of the worm... it could be larger than the bearings................

As for bearing area, there is always the spur gear and rack drive.... potentially far less pressure area than even the rack and worm.

wierdscience
12-11-2012, 09:01 AM
I forget what it was called,but didn't one of the Planer mfgs use a hypoid rack drive for table movement?Seems that would be close to a worm drive rack.

lowcountrycamo
12-11-2012, 09:03 AM
Nice outside the box thought.

Another possibility is that there is no law giving the maximum diameter of the worm... it could be larger than the bearings................

As for bearing area, there is always the spur gear and rack drive.... potentially far less pressure area than even the rack and worm.

When I first picture the train I imagined a worm as large as worm wheels usually are. A large worm would give greater accuracy.

This is the only pic I could find of worm/rack

http://etc.usf.edu/clipart/26900/26962/rackwormgear_26962_sm.gif

topct
12-11-2012, 09:28 AM
I have no idea what I was trying to do with this. The 20tpi rod will mesh with the 20 teeth per inch rack but then how to support the rod. An old printer gear made a good idler though.

http://img.photobucket.com/albums/v78/topct/Rackandgear.jpg

Toolguy
12-11-2012, 10:41 AM
That setup could provide for very fine feed of the rack if the gear were free to turn on an axle and the rod were in a stationary housing with bearings.

RLWP
12-11-2012, 10:52 AM
How fast is the engine going to be going then? Not the speed along the track, the rotational speed

Richard

Optics Curmudgeon
12-11-2012, 02:25 PM
He's talking about train as in gear train, not mode of transportation.

loply
12-11-2012, 02:34 PM
Is there any advantage of a worm rack over a leadscrew and nut?

topct
12-11-2012, 03:15 PM
That setup could provide for very fine feed of the rack if the gear were free to turn on an axle and the rod were in a stationary housing with bearings.

And you could place the screw at a 90 to the rack.

Now I remember what I was trying to do. A rack and pinion steering box for a large RC car. I would use the idler gear to turn the feedback pot. The parts were much to small and crude for the size of car so I scrapped the idea.

Peter S
12-11-2012, 05:26 PM
Here is William Sellers spiral drive for planer. I am pretty sure it engages with a rack under the table.

http://img.photobucket.com/albums/1003/PeterS/sellersplanerdrive.jpg

lowcountrycamo
12-11-2012, 06:14 PM
Is there any advantage of a worm rack over a leadscrew and nut?

The only advantage I could imagine would be more accurate movement and better resolution. But I don't know if that is possible. And I should assume not, or rack/worm would exist in more machinery.

The Artful Bodger
12-11-2012, 07:26 PM
Presumably a greater load could be handled for the same size rack as the pressure is not concentrated on one or two rack teeth.

kf2qd
12-11-2012, 08:34 PM
The worm is at an angle to the rack. The rack is just a normal straight cut rack at 14 or 20 degree pressure angle. The worm and rack has the same problem as a worm and wheel. Not very efficient. All motion of the worm is rubbing on the rack. The contact are is constantly moving so wear would pretty even.

joe51
12-15-2012, 12:15 AM
Is there any advantage of a worm rack over a leadscrew and nut?


The only advantage I could imagine would be more accurate movement and better resolution. But I don't know if that is possible. And I should assume not, or rack/worm would exist in more machinery.

I would think that as long as both systems had the same pitch, their accuracy and resolution would be the same, at least ideally. The disadvantage of a worm/rack system is that the worm would apply side pressure to the rack and deflect it. Any deflection in the rack would then allow it to move laterally and therefore lose accuracy. A leadscrew and nut is inherently self-centering and there is no side load generated and therefore no deflection.

Paul Alciatore
12-15-2012, 04:17 AM
I think the answer to the question is that a length of threaded shaft, even with Acme or square threads, is less expensive to manufacture than a similar length of rack. Likewise, the nut, Vee, Acme, or square, is also less expensive to manufacture than a large worm would be.

Threaded rod or precision lead screws are made to very high levels of accuracy so there really is no advantage in accuracy. Again, it is probably easier (cheaper) to achieve a given level of accuracy in a screw than in a rack. Rack teeth would need to be cut at precisely spaced intervals and this would require an even more precise lead screw.

Another point is wear. A simple rack/worm combination would have a single line of contact at each tooth. This would tend to produce wear that is concentrated at the center of the rack teeth. A screw/nut combination has a large area of contact.

Economy wins. Case closed.

Forrest Addy
12-15-2012, 06:35 AM
Worm and rack table drives were not uncommon in the days of planers. It was a stiff drive, smooth, simple and not difficult to manufacture.

Generally, the worm was inclined across the rack face to both distribute wear across the rak teeth and simplify bearings; the worm bearing housings could straddle the rack. Another advantage is the pitch, number of worm starts, helix angle could be juggled to tuck the motor up near the bed behind the column. Very compact. I have a book somewhere on planers, shapers, and slotters that has a fine illustration of the Sellers drive.

The Sellers planer table drive consisted of the table rack driven by the worm which was direct coupled to the motor. Two active parts, electric reverse. No complicated double helical gearing, hydraulic cylinders and valves, no belts and pulleys flapping near your shoulder ready to snatch off your arm. The Sellers drive main problem was it was inefficient to the point that a period of capacity operation would warm the table.

Hopefuldave
12-15-2012, 08:12 AM
Some of the LeBlonde Heavy-Duty lathes (from the 40's, 50's?) used a worm and rack for the tailstock quill - the quill had a rack cut into it underneath (teeth square to the lathe axis) and a matching worm set at the helix angle with a micrometer dial - two advantages: 1) lots of inherent friction so the tailstock quill didn't need to be locked every time it moved; and 2) the tailstock quill could be hollow allowing work to be passed through. I think (memory fails me) the tailstock took the same taper as the spindle nose, so anything that passed through the headstock would pass through the tailstock too?

I think I've seen this on some of the larger English lathes from that era, as well.

Dave H. (the other one)

kf2qd
12-15-2012, 02:58 PM
Some of the LeBlonde Heavy-Duty lathes (from the 40's, 50's?) used a worm and rack for the tailstock quill - the quill had a rack cut into it underneath (teeth square to the lathe axis) and a matching worm set at the helix angle with a micrometer dial - two advantages: 1) lots of inherent friction so the tailstock quill didn't need to be locked every time it moved; and 2) the tailstock quill could be hollow allowing work to be passed through. I think (memory fails me) the tailstock took the same taper as the spindle nose, so anything that passed through the headstock would pass through the tailstock too?

I think I've seen this on some of the larger English lathes from that era, as well.

Dave H. (the other one)

They were still using that system into the 80's on the Regal Lathes. I ran a LeBlond Tape Turn Regal that had that. The Also put a 2 speed drive - just behind the handwheel was a large knurled piece that slid back and forth for high and low speed. Large spindle quill and a pass through. On that machine the spindle had a 2-1/4 hole and the tailstock had a 1" hole. I would love to have one of those Regals...

oil mac
12-15-2012, 03:15 PM
I personally believe William Sellars with his spiral drive to the planing machine table, was one of the finest examples of engineering genius.

.RC.
12-15-2012, 04:08 PM
Friction and wear is the big killer... Gears roll, worms slide...

wierdscience
12-15-2012, 11:07 PM
I personally believe William Sellars with his spiral drive to the planing machine table, was one of the finest examples of engineering genius.

Ah,that's the one I was thinking of.It had two advantages over rack and pinion drives.Number one it was quiet and number two there was no spur gear chatter to telegraph into the workpiece finish.I believe it was Herring Bone gear racks that finally made them obsolete.

A fine example of a Seller's drive machine was this Gray Planer-
http://vintagemachinery.org/mfgindex/imagedetail.aspx?id=4512

Forrest Addy
12-16-2012, 12:27 AM
Wanna worm and rack example? Look at your adjutable wrench.