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John Stevenson
09-20-2011, 04:03 AM
Got some friends who are big into vintage car racing with things like Alfa's and Alvis's.

Blowers for these are very, very hard to source and consequently very expensive. The VMCC governing body is quite lenient over what or what can or cannot be fitted and basically it has to look period.

Two ways to do this, start from scratch and make the lot, case will have to be patterned and cast anyway to get the period look, off the shelf gears and the rest made to suit.
Second way is to get a modern Merc Kompressor off Ebay <>£250, gut it and make a new case round it.

Stumbling block for me at the moment is finding something out on the design of the rotors. Seen the Wiki pages with the diagram but also read that this isn't the mathematically correct shape and this is borne out from some pictures I have seen of the rotor shape where the head is a semi circle.

Soooooooooooo any links please to any papers on two lobe design so I can explore all options ?

The Artful Bodger
09-20-2011, 04:34 AM
Rootes type blowers?

uncle pete
09-20-2011, 04:36 AM
John,
I'm not going to be any help with a direct link for information but I can suggest a few search terms. Try GMC 6-71 Superchargers or for modern ultra high performance maybe "Screw Compressor" I could be wrong but I believe these were banned over here for drag racing. I never did hear exactly why. That would be a damn interesting project. Wish I could be more help.

Pete

BWS
09-20-2011, 04:38 AM
Check your PM's.

MrSleepy
09-20-2011, 06:01 AM
Hi John

These may help.

http://researchbank.rmit.edu.au/eserv/rmit:6362/Ilie.pdf

http://www.holley.com/data/TechService/Technical/Supercharger%20Tech%20Info.pdf

http://www.jeperformance.biz/BlowerDesign.htm


Rob

Metalmelter
09-20-2011, 06:34 AM
I know these guys do it all - maybe they can help. There's some technical info that you can download too.

http://www.blowerdriveservice.com/

Cheers!

willmac
09-20-2011, 06:37 AM
John -

The vintage cars probably used Roots type blowers, which are noisy, not very efficient and don't really compress air - they just shift large volumes very quickly. If the rules allow, a modern screw compressor should be more efficient and they actually do compress air. You could make the housing look like a vintage blower so that may be an ideal solution.

The rotor profiles are quite complex and it is the profiles that make them work, so concentrating on getting them right is essential. As well as the geometry of the rotors, there is the equally complicated problem of how to make them. Specialised milling and grinding machines are used for this. The clearances (rotor to rotor and to housing) need to be of the order of a few microns. This means that metrology for these parts is also quite complex and specialised machines may be used (I was involved peripherally with this in the distant past). Given this, you might be better off (as you suggest) finding a reasonably common set of rotors that you can source second hand and building the rest around that.

Holroyds (in Milnrow) are the place to go for screw compressor rotor making and inspection machinery.
http://www.holroyd.com/holroyd-precision/machines/machines-overview.php
Just having a look at their web site may be helpful.

If you use Holroyd as a search term with screw compressors, you will find lots of papers that should help.

Here are a few to get started:

http://www.staff.city.ac.uk/~sj376/stosic99.htm
http://www.staff.city.ac.uk/~ra601/interfer.pdf
http://books.google.com/books?hl=en&lr=&id=Ax4NALZ_UMsC&oi=fnd&pg=PA1&dq=holroyd+compressor+rotor&ots=50cu4V8Oxg&sig=Q4cWYmZsCOUhRkXOUalpCzPJBwo#v=onepage&q=holroyd%20compressor%20rotor&f=false

Note that the papers do contain some generalised rotor drawings.

There are patents in this area you may need to be careful of if you are going into production!.

small.planes
09-20-2011, 06:43 AM
Got a couple of books on this sort of stuff, Ill dig them out and drop them round.
how urgent?

Dave

vpt
09-20-2011, 08:04 AM
I'm gonna watch this! Crazy tight opposing tolerances are exciting!

I'd look for a modern unit and modify the housing to make it look how you want.

Housing are easy to work on or reproduce.

http://img238.imageshack.us/img238/2608/projectsupercharger022hv6.jpg

http://img377.imageshack.us/img377/8441/projectsupercharger006lv1.jpg

J Tiers
09-20-2011, 08:12 AM
I'm gonna watch this! Crazy tight opposing tolerances are exciting!



how ABOUT them backlash specs..............:eek:

vpt
09-20-2011, 09:09 AM
how ABOUT them backlash specs..............:eek:


There is at least a 1/1000 of a degree leeway there, should be easy to work out. :D

Willy
09-20-2011, 10:23 AM
how ABOUT them backlash specs..............:eek:

Not sure how backlash is accounted for on the straight cut gears in VPT's post, perhaps he can shed more light on this.

I've done several GM Rootes type blowers in the past and they use a very novel approach.
Rotor lobe to housing, and rotor lobe to lobe clearance varies depending on where of course it's measured, but does go down to about .003". Been a while but I think there are about 8 or 10 critical measurement points to take, and this is on a medium speed, 2100 rpm 2 cycle Detroit Diesel. Mind you they are overdriven as Detroits do like LOTS of air.

Essentially rotor gear backlash is held within .0005-.0025". The rotors are then timed to one another to establish running clearance by selecting shims that go behind one of the helical gears to move it in or out relative to the other gear.

Bill736
09-20-2011, 12:46 PM
The statement that Roots type blowers "don't really compress air" is incorrect. Of course they do, and modern dragsters use them to compress atmospheric pressure air to 50 psig air in the intake manifold of the engine.

John Stevenson
09-20-2011, 02:20 PM
General replies,
Sorry should have said Rootes Blower, I know these are not the best things but they were original and do improver the performance and with the Vintage guys it's all about looks more that wring the last ounce out of them.

Rob, thanks for that last link, some good information there.

Wilmac, thanks for the links but screw compressors are definitely out doe to the complexity of manufacture.

BWS still digesting that link.

Dave, Small planes, be interested if it had geometrical design in it, no rush, it's an over winter project.

Most of the links I have found are just advertising or mumbo jumbo on amounts of air moved.
I'm more interested in basic design.

willmac
09-20-2011, 02:22 PM
There is no change in the volume of air as it passes through a true Roots blower, and therefore the air is not compressed in the blower. In comparison, a screw type compressor or a vane type compressor DOES internally compress the air that passes through it. The fact that the Roots blower does not compress air internally does not mean that it is incapable of delivering a positive pressure ratio from atmospheric to inlet manifold.

There is a significant difference in the performance of screw and Roots type. You need to choose based on the requirements. A dragster doesn't need to be thermodynamically efficient.

TGTool
09-20-2011, 02:35 PM
There is no change in the volume of air as it passes through a true Roots blower, and therefore the air is not compressed in the blower. In comparison, a screw type compressor or a vane type compressor DOES internally compress the air that passes through it. The fact that the Roots blower does not compress air internally does not mean that it is incapable of delivering a positive pressure ratio from atmospheric to inlet manifold.

There is a significant difference in the performance of screw and Roots type. You need to choose based on the requirements. A dragster doesn't need to be thermodynamically efficient.

I'm no engineer and can't argue formulas for what goes on in a compressor. However, what it reminded me of was a hydraulic gear pump which also does not compress anything even if the fluid were compressible, but it does develop a positive pressure between inlet and outlet. Somehow. :D

An engineer and fluid power expert once explained to me that the difference in pressure is also not incremental from one tooth cavity to the next as the pump is sweeping fluid. From research, as soon as the advancing tooth passes the pump housing wall it instantly has full outlet pressure. Curious. Of course air is different since it is compressible.

The Artful Bodger
09-20-2011, 03:42 PM
Air is compressed in a Rootes type blower:-
http://upload.wikimedia.org/wikipedia/commons/thumb/7/7b/Roots_blower_-_2_lobes.svg/250px-Roots_blower_-_2_lobes.svg.png

In this illustration the air in chamber 'c' is undergoing compression..

willmac
09-20-2011, 04:43 PM
John -

If you are looking at a true original Roots type blower, the rotors are straight - i.e. no helix. I have no experience of making small Roots blowers, but I have worked with a company that made big blowers. The rotors were made on a CNC planer (yes really). I would imagine that you could treat them like gears and make them on a mill with a 4th axis.

willmac
09-20-2011, 04:56 PM
Artful Bodger -

Chamber C is open to the inlet manifold. The pressure in the manifold rises as each chunk of air is delivered to it, assuming that the rate that the air leaves the manifold is less than that which the blower delivers.

The analogy that TGTool gave is useful.

Gravy
09-20-2011, 07:43 PM
If you want to get really picky about it, any device that mechanically moves air from one place to another compresses it. Wave a fan blade through the air - the air in front of the blade will be more compressed that the air behind the blade. It's all about pressure differentials. So there!:p ;)

vpt
09-20-2011, 08:18 PM
Willy, I didn't do anything with the rotors or gears. I just did the housing work and rebuilt the snout. The gears were tight, I couldn't see or feel any looseness at all. Didn't want to play with that stuff one bit. :EEK:

Bob Farr
09-20-2011, 09:26 PM
*** The VMCC governing body is quite lenient over what or what can or cannot be fitted and basically it has to look period.

Two ways to do this, start from scratch and make the lot, case will have to be patterned and cast anyway to get the period look, off the shelf gears and the rest made to suit.
Second way is to get a modern Merc Kompressor off Ebay <>£250, gut it and make a new case round it. ***

Take a look at Rotrex centrifugal superchargers. It's not the traditional lobed design you're looking for, but it is a very compact unit which coundl be concealed inside a more convincing period correct casting if needed. Just a "plan C" to consider.

http://www.rotrexsuperchargers.co.uk/Rotrex/index.htm

I'm looking forward to how you work this out. Best regards.

Bob

flylo
09-20-2011, 11:18 PM
I have a 2 cyl french Citereon engine converted for aircraft use running a gear reduction prop hub & single carb not making enough power so next step is a Mitsubishi turbo off a BMW. This fed 3 cylinders on the BMW & is just the right size. It's the smallest turbo I've ever seen. Should make 70HP+ when done. Probably a different cam. Had dual carbs,had fuel injection went to a single Harley carb, Trying to make these idiot proof.
Also need a reasonable source for cores of this engine if anyone can hekp. Not trying to steal your thread,sorry.:(

John Stevenson
09-21-2011, 03:29 AM
I am reasonably familiar with the Roots try blower as these were fitted to a few of the two stroke diesel trucks, bit ironic really in that we scrapped loads just for the alloy :confused:

Most were 3 lobe units but I have seen the odd 2 lobe one.
It's just something that's kicking about in my head as a sort of retirement project.
Given that it's an interesting project making the pattern and getting it cast, I'm lucky in that we have a local foundry where the boss is very helpful with interesting projects.

Add to this these guys with the cars don't skimp, many employ people to build their engines etc. There are a high percentage of doctors, solicitors and businessmen who own these and it's a bit of a closed shop.

Do a nice job for one and you get recommended.

Can't see a problem with the case, OK fair amount of work but most of it is in the pattern / casting / visual looks department.

Gears are off the shelf helical units and if one is fitted on a vernier you don't have to worry about tight machining tolerances, bearings standard etc.

The problem I have is that the diagrams I have seen for the lobe design with dumpy curves on the top.

http://www.jeperformance.biz/jepimage/roots.GIF

Do not match up with the full circle designs I have seen in real life.

http://www.sdrm.org/roster/diesel/emd/history/roots-1.jpg

looks like I'm going to have to get small son to draw this up in Solid Edge and see how it fits.

Plan B is to get one off Ebay and see how usable it is to use the rotors out of that, might be the better plan given it will cost more to make two than to buy a unit complete with gears and bearings that mesh OK.

bob ward
09-21-2011, 04:20 AM
This is a wonderful example of custom blower work, the aluminium castings are a work of art. The car is a 28 or 29 Hudson remodeled along the lines of a blower Bentley, the engine was moved back in the chassis to allow room for the crank driven supercharger. This Hudson was built in Melbourne Australia maybe 15 years ago and now resides in the Hudson Museum at Shipshewana Indiana.

http://i237.photobucket.com/albums/ff28/sirrobertthegood/IMG_4238.jpg

http://i237.photobucket.com/albums/ff28/sirrobertthegood/IMG_4239.jpg

uncle pete
09-21-2011, 04:28 AM
John,
For what it's worth the drag racing type now use a teflon? wiper that fits in a slot cut on each lobe. If a bearing starts packing it in then you usually catch it before it wipes out the outer case. Plus they can be cut really tight to the case wall. Might be easier to machine the lobes to looser tollerances if you used something like that.

Pete

willmac
09-21-2011, 06:26 AM
John -

A number of thoughts about Roots rotor blower shapes.

There are claims that the shape is, or can be based on a cycloid form. This makes a bit of sense and the reference (see last paragraph) shows how the form is generated (that should make this thread run and run!!). I'm not convinced that the form has to be cycloidal but it is worth a bit of thought.

http://mysite.du.edu/~etuttle/tech/cycloid.htm

More advanced screw type compressors use different shapes for the two rotors, male, and female. The rotation of these shapes causes the compression of the air. The rotors are designed such that there is a 'sealing line' formed as they rotate and trap air. I know you don't want to use a screw compressor but I think you can apply some of the theory that has been developed to make your Roots type blower work a bit better. See this paper for more info:

http://www.springerlink.com/content/u45k3478xu500771/fulltext.pdf

The second diagram you show has helical rotors (and hollow ones at that). Making them helical obviously means that you will need to think about whether the profile you use is defined normal to the helix or to the axis of the rotor.

I think it was Eaton that developed better performing Roots type blower with helical rotor forms for ordinary production car use. I think they also introduced the idea of an 'Abradable Powder Coating' which quickly wears away in areas where clearances are too small (microns here) leaving as close as possible fit between rotors and housing. I don't know what this coating is, but it might be worth investigating it as a way to get good performance without spending a fortune on the kind of machines that can bore to a few microns along the length of a housing.

small.planes
09-21-2011, 07:16 AM
I cant find it now, but I remember some funny looking gears made of white plastic.
They looked a lot like blower parts...
I thought it was a gearotic related thing, but my googlefu is weak today.

Dave

John Stevenson
09-21-2011, 07:36 AM
Aahh,
You mean these one tooth gears ?

http://www.youtube.com/watch?v=0yVngc21q9A

vpt
09-21-2011, 08:02 AM
Ha, thats pretty neat.


John I would seriously consider getting the unit off ebay like you mentioned and then making the case look how you want it to. The one I have pictured is a eaton M62 for 1.6liter honda motors. The unit themselves, used, are very cheap. You could even leave the stock case and build an outer cover for the case to make it look period correct and then add fluid between the cases for cooling if they are willing to do something like that. Many of my customers are open to my suggestions, very few have a set idea on exactly what they want.

Also like mentioned if they want better power you could hide a centrifugal supercharger in a roots case.

I love SC's! I have 3 of them, the one pictured earlier on a car and two in boxes yet waiting for a car. I have been wanting to throw one on my daily driver but I don't want to spend the $500 for another tunable ECU.

small.planes
09-21-2011, 01:34 PM
yes, the 2 tooth paradoxical gear set is what I was remembering :)

Dave

Asquith
09-21-2011, 04:57 PM
http://i3.photobucket.com/albums/y54/Asquith1/July%202011/Roots.jpg

There were some quite big Roots’ blowers made in the US and UK long ago. Thwaites & Carbutt made this one in 1877 with 25 ft diameter rotors, for mine ventilation. More information here:-

http://www.gracesguide.co.uk/Thwaites_and_Carbutt

The ‘tooth’ form wasn’t very sophisticated, and clearances were huge, at 1/8" ;)

jimdanp
09-21-2011, 05:41 PM
Check patents.
These rotors maintain clearance .002/360 degrees.
A roots type blower is a positive displacement pump and not a compressor but if turned fast and volume is great enough it will have a positive pressure.
BTW I think the dragster people use nylotron(sp) not teflon.

http://jdpiazza.com/pages/rootsblower/DSC_0564.jpg
http://jdpiazza.com/pages/rootsblower/DSC_0572.jpg
http://jdpiazza.com/pages/rootsblower/DSC_0573.jpg

Charles P
09-22-2011, 03:09 AM
John

I don't want to dampen your bonfire but a couple of points.
Since Alfa and Alvis are cars I assume that you meant VSCC, not VMCC. VMCC is for two wheelers.
Secondly twisted rotor Rootes blowers are very post war and are not acceptable for VSCC club competition. I don't think that teflon tipped rotors are allowed either for the same reason.

Charles
(who is still making a proper replica Cozette vane supecharger but it'll take a while!)

John Stevenson
09-22-2011, 04:04 AM
Charles,
Could be VMCC as they spend more time on two wheels :D but yes you are correct, I just grabbed an acronym out of the air that sounded right.

Thanks for the information on the helical rotors, I didn't know that but short of buying a S/H unit to cannibalise I would not be making helical rotors, far to much work involved.

I'd like to take a moment at this point to thank all the people who have replied both in this post, by PM's and email, many with articles attached which I need to go thru.

For what was a 'blue sky' simple enquiry that may not go anywhere other than a design project I'm over whelmed at the help and support I have got from this post.
I expected some replies hence posting here but the value and content on this subject has out weighed most of my previous requests over the last few years [ with the exception of the hobbing generation :D ]

One site I did find especially interesting was this one sent to me by a guy called Mooselake who's not on this forum.

http://www.netduke.net/2009/12/diy-roots-blower-blower-case-and.html

You need to click the 2008 page at the side then work up in date order.

So again thanks for all the info and I'll report back when I have read all the stuff that's come thru, two articles came this morning.

Black Forest
09-22-2011, 10:52 AM
Someone has hacked John Stevenson's account. The real John would never write something so elequent and complimentary. The real John is probably handcuffed to his Bridgeport while the imposter throws us off.

John did you read my E-mail?

uncle pete
09-22-2011, 11:53 AM
Yeah you just might be right. Think back, He recently posted about buying/trading for a slotting head for the Bridgy, Somewhat implied it would be a useful machine Swept the floor, Hired on a apprentice who he wanted to start off on the right foot. Then again it could be faulty meds?:D

Pete

John Stevenson
09-22-2011, 11:58 AM
Damn,
Fortunately I manage to tip the wimpy Bridgeport over and get the cuffs off.............

Wot email ? nothing showing here,
Presume you put a subject line in as all empty subject lines get dumped.

[EDIT] Still not got the slotting head, should have been delivers on Tuesday but didn't show, not worried the guy is 100% but dead busy.

New apprentice is ace.

First question on Monday morning "What is the first thing you do on entering the shop ?"

His reply was "Make coffee ? Sweep up or listen to war stories" God he's going to fit in well.

I asked him "Do you know what weight one person can life under current H&S laws ? He replied no so I told him to fit the big 10" dividing head on the mill.

alsinaj
09-23-2011, 11:36 AM
Sir John: could you not use the Art Fenerty facet-cut gear method to make SC rotors on the mill?

Peter.
09-23-2011, 12:47 PM
Take a look at Rotrex centrifugal superchargers. It's not the traditional lobed design you're looking for, but it is a very compact unit which coundl be concealed inside a more convincing period correct casting if needed. Just a "plan C" to consider.

http://www.rotrexsuperchargers.co.uk/Rotrex/index.htm

I'm looking forward to how you work this out. Best regards.

Bob

I was going to suggest this - except I was going to say Procharger as they use less RPM and a larger compressor.

jrude
09-23-2011, 03:55 PM
The old ball drive Paxtons were built back in the 30's and installed on various automobiles as OEM type products all the way through the 60's. They are still available today from a place in California named Paradise Wheels who bought out the old SN series rights a long time back. An old SN92, SN93, or SN2000 are about 20 years old and might look period, depending on the period. Also, the McCulloch supercharger was the same as the Paxton ball drive but much older in vintage. I suspect you could buy a 20 year old SN93 and make it look like a 60 year old McCulloch pretty easily.

Yow Ling
09-23-2011, 05:09 PM
Hi John, check out the Toyota 4age or 4agez superchargers they are sized around 1600cc. Also Wankels rotary engine was first designed as a supercharger, so that should sort out any eligibility problems using a trochoid and rotor

John Stevenson
09-23-2011, 05:21 PM
Right the Ebay route isn't going to work because the VSCC club have banned helical and screw superchargers, suppose that also counts on putting a rotary in a different case.

Might as play to the rules, lot of work otherwise.

Anyway long short is I have literally pages of information from various people, some who don't mind being mentioned and some who would like to remain in the background for certain reasons.

I now have three different layouts for 2 lobe rotor design in various forms but the highlight was an email from a guy who had built one for a vintage ERA who has sent the engineering drawings of the rotor design, fully dimensioned with arc centres etc so it can be scaled to any size. Again a different shape to previous ones I have seen but he has asked that the drawing not go any further and I have no problem with this..

As a further aside now that Art Fenerty of Gearotic and Mach3 fame is back from his holidays in Amsterdamage I have forwarded some info on these designs for him to peruse and we may find at a later date that Gearotic mill be able to generated rotors.