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Sprockets for the Sherman series tanks.

When I began this Sherman project I started with the sprockets and track. I wanted to be sure I could actually make these darn things. The use of a CAD program can be of tremendous help when doing the layout work although a sharp pencil and paper can be as good if you take your time. I used "Turbo Cad" by Imsi. A free basic version of this program can be downloaded from their web site at http://www.solutioncity.com/tc_detail.php3

There is an upgraded version of this program also available for a very reasonable price. I highly recommend it for model drawings. The program will allow for precise printing of a drawing on inkjet or better printer (You draw a 10-mm line and it will print a 10-mm line.) I recently run across this program at a major electronics and music warehouse (Big chain of stores) for about $45.00.

A major point of consideration is the measurements for the sprocket. I can help you on the measurements for the Sherman sprocket. The sprocket used for the Sherman can look quite daunting until you begin to compare it others such as the Tiger tank.

As you can easily see in the two pictures above. (Thanks to Mick Cockerill for the picture of the German tank sprocket.) The amount of work needed to reproduce the German type sprocket as compared to the American style is quite a bit. The German type hub has some fairly integrate spacing between the outer ring and inner hub. The Sherman used different sprocket tooth plates. Pete Harlem's "Modeler's guide to the Sherman." list five different variants of the sprocket plate. I would highly recommend Mr. Harlem's book for any of you considering building a Sherman series tank. The cost of the book is around $25.00 USF and well worth every penny. On page 68 of his book, there are 1/15 scale drawings of the sprocket plates. The info could be utilized in your own drawing and used in a master transfer pattern to make your own sprocket plate. I used the 'Type 2 revised' for my project. This sprocket can be seen in the photo on the left.

One difficult part of making this sprocket on a drill press might be the grooves. A jig and angle vise may help in this area. The simple plate style (Seen below) would of course be a bit easier to make especially if making it from steel plate on a drill press.

I made my master pattern from a 1/8 piece of plywood purchased at the hobby store. The pattern was printed on a printer then glued to the wood. A scroll saw was used to cut the pattern, a trip to the beltsander for the final trimming and finishing operation. The inside of the tooth ring was cut out using the scroll saw. The same pattern could be cut using a hand Coping saw. The angled slots were made with the motor tool with a cylinder-cutting bit. I cast my sprockets in aluminum but they also can be fashioned from steel plate (3mm or 1/8 inch) on a drill press and then hand filed to final shape. The hubs (two pieces) between the sprocket plates are also cast in aluminum. The sprocket tooth rings are then spot welded onto the hubs and then put together. The retaining bolts that hold the tooth ring onto the hubs can either then be drilled, tapped onto the hubs or if you prefer the head on a small scale hex bolt can be simply epoxy to the teeth ring. Below you will see the cast version before detailing on the right and the master pattern model on the left.

If you decide to make your sprocket plate from steel plate, a bench top drill press would be quite handy for this. Dave Spring of 6th Armor authored a piece about building sprockets on a drill press a few years ago for the Recon Report newsletter produced for the Bay Area Tankers club. (See end of this article for Dave's great article.) For those looking for a real challenge, a hand held power drill would also work. The idea here of course being to paste a pattern onto the plate then drilling holes around the pattern. The holes would need to be fairly close together for a small file to be inserted. You would use the small file to cut through the hopefully small gap between the holes.

I have on rare occasion used a metal cutting scroll saw blade to make these cuts but you must use plenty of oil to keep the blade from dulling so quickly. On 1/8-inch steel the blades will not last long. To give you an idea, I think I used 6 blades to make a six inch cut.

A hand held side grinder would be better suited for this job in cutting the rough outline out, then using the bench grinder to further remove unwanted metal from the pattern. A belt sander would be a great aid in the finishing operation of the final cut.

Above is the master pattern I printed for the sprocket tooth plate. I can send a version of this file by request either in the native format of Turbo Cad (TCW) or Drawing Exchange Format (DXF) via email if desired.

The master pattern used for the hub was actually a piece of plastic (See picture ) I found in my workshop, as my luck would have it, it turned out to be the perfect piece in size for what I needed. A few changes such as filling in the middle in with putty were all that was needed to be ready for the casting operation. Still keep in mind the 6% rule of metal shrinkage when looking or making the proper hub size. Another idea would for the hubs would be PVC pipe that is just a bit larger in diameter than needed purchased at any hardware or plumbing supply store.

The center portion of the pipe would need to be filled in, plastic filler (Bondo, or any automotive body repair putty could be used for this.) even Plaster of Paris could be used. Another idea would be to use round hand rail wood stock. Various sizes can be found at most lumberyards. Due to casting problems (see 3rd part of casting article else where on Sherman Builders) the hubs are cast in two pieces.

Note the transmission cover located behind the sprocket. Also take note of the retaining bolts for the sprocket tooth plate. These bolts on my model sprocket will only be for scale appearance as the sprockets are welded to the hub.

The sprockets should be supported with a roller bearing. The bearing can either be located in the transmission housing cover or the inside of the chassis wall. Remember the shaft protruding from the transmission housing into the sprocket will need to be immobile in the hub itself as this is the shaft that will be linked to the transmission system. Securing the axle in one half of the hub before attaching the two hubs together may be the best way to do this.

Some late thoughts

Some thoughts about using aluminum for the sprocket plates, as many of you know this Sherman project of mine is a living sort of thing. I still continue to work and make changes in the way I make something or the material I use to make a certain part. I try to update the website when I do something different as I'm in firm believer of sharing all that I have learned about this small segment of the hobby world.

An E-chat with Harold Thompson brought some news that I had not been aware of. Our E-chat conversation printed below may prove of interest for those interested in building a tank. I would like to thank Howard for allowing me to post this info.

The following is an excerpt from an E-chat I had with Howard Thompson.

My question concerned the force against the teeth of a sprocket for a model.

Howard's reply:

The force at the sprocket teeth to drive a 1/8th size 100 pound model at a sustained speed on a hard surface is about 5.0 pounds. During acceleration or climbing a steep grade it can be twice to three times 5.0 pounds, depending on the surface. So for a worst case situation let us use 15 pounds force. If the scale cross sectional area of a sprocket tooth is .03 inch square then the shear stress in the one tooth will be about 500 psi.

This is assuming that sometime, due to misalignments, only one tooth will be driving the model. Shear strength in most common aluminum sand casting alloys is 20,000 to 30,000 psi. This would indicate a first order determination says you should not have a problem. This is also true for the pin. Most aluminum wrought alloys that are heat treated to the T6 condition also have shear strength of 20,000 to 30,000 psi. Therefore, if the track pin is scale or 1.250/8 or .156 dia. Its cross sectional area is approximately .02 inch square, and if one pin is carrying the 15 pounds the stress will be 750 psi. What I see as the larger problem in the reliability of any track is the smoothness of operation and the compatibility of the surfaces in contact.

The smoothness of operation is primarily due to clearances and the alignment of the pinholes. The compatibility of the materials in contact determines the friction between parts in a non-lubricated dirty environment. Unfortunately aluminum against aluminum is the worst case. The friction coefficient between aluminum surfaces is three times greater than that of steel against aluminum or steel against brass. This can result in galling of the aluminum surfaces under sustained force, which increases the load on all components of the drive. I have suggested to others using aluminum sprockets against aluminum shoes to have the parts anodized. This gives the parts a hard aluminum oxide coating, which is quite ware resistant. Particularly if the �hard coating� process is used which results in a much thicker coating.

I think in addition of his great suggestion about anodizing parts, you can also see that using two different types of metal will prevent or at least alleviate this problem. Steel and aluminum working together should have no problems; however the question is which two of the components between the sprockets and track do you use metal. The core of the problem lies between the sprocket plates and the end connectors on the track that mesh with the sprocket. Both at this point are made from aluminum, (see late addition to track making file) after careful thought about the subject; I have decided to forge on and continue making both from aluminum. What's a world without chances! CHUCKLE!

Seriously though, the track end connectors are a difficult item to make from steel, at least without a specialized punch machine designed for such operation. The resin type of connectors previous mentioned was later deemed not to be as strong as I hoped for the project, although for a static display they prove to be quite ample. The end result was to make these from aluminum bar stock, a very easy method that produced a nice looking part for the model. The sprocket tooth plate is the only other part for consideration in steel. While the fancy tooth style I have cast in aluminum may also prove to be a nice piece for static display, the final model may have the tooth rings made from steel plate.

Dave Spring of Spring's 6th Armor page kindly granted permission for Sherman Builders to post the following article. His article concerning sprocket making first appeared in Recon Report #5 in Jan 1995

(Before we begin I must state that for the purpose of keeping this article short, simple and to the pointnot all definitions, or descriptions for gear cutting will be used. Only those needed to make a sprocket and mating track blocks that function well as a unit.)

Minimum tools required are a calculator, a vernier caliper capable of measuring in thousandths and a drill press with a table square to the head of the drill press to maintain accuracy of the parts to be made.

Your hole or tooth spacing measurements must be calculated to four decimal places to keep cumulative error small.

Example

If you were to layout a sprocket's circular hole pattern and the spacing was only off .005" successively from the first to the last hole or tooth, this amount would be multiplied by the number of holes. On a Sherman sprocket with 13 teeth it needs 26 holes. 26 X the .005" error = .130" - that�s over 1/8' error between the first and last tooth. You can toss that part out on the target range for future use.

To use the formulas you will first need to know two things obtained from research on the full-sized vehicle:

1) PITCH DIAMETER OR P.D.,

2) NUMBER OF TEETH OR N

This in order to determine two measurements: First is the diameter on which the hole-centers are to be placed for the sprockets. Second is the center to center distance of the track block pins. The following formulas may be used in any scale except the one with the asterix:

#1 P.D. = (N x C.P.) � 3.1416
#2 O.D. = (N + 2) � D.P.
#3 C.P. = 3.1416 � D.P.
#4 D.P. = N � P.D.
#5 C.T. = [SIN(90 � N)] x P.D. IN YOUR SCALE

Definitions:

D.P. is the diametral pitch

P.D is pitch diameter*

O.D. is the outside diameter

C.P. is circular pitch

C.T. is chordal thickness-this will be our center distance of the pins on the track.

Example #1:

M5A1 Stuart light tank sprocket 1:1 scale 13 teeth- 22.8" P.D. -.57017 D.P. Once these are known you can determine the P.D. in the scale you wish to model in, and the pin spacing. Let's try it out using 1/9 just as an example.

22.8 � 9 =2.5333 This equals the diameter the holes centers will be placed on. Also this number becomes your scale P.D. to be used with formula #5 to find the proper pin spacing on the track block to mate with the sprocket.

C.T. = [sin(90 � 13 teeth)] x 2.5333" which equals .3053" Please don't forget that on the early type U.S. sprockets 13 teeth require 26 holes spaced evenly at .3053" Some more examples using the same tooth, pitch dia. and diametral pitch:

13 teeth - 22,8 P.D. - .57017 D.P.

Scale:C.T. (track pin spacing)

1/10 = 2.28 ---------------.2748

1/8 = 2.85 ---------------.3435

1/5 = 4.56 -------------.5496

1/9 = 2.533 --------------.3053

1/6 = 3.8 ----------------.458

EXAMPLE # 2: Sherman M4A1/2/3

13 TOOTH SPROCKET 25.038"

.51921 D.P.

Scale P.D. C.T. (Pin Spacing)

1/10 = 2.5038 -----------------------.3017

1/9 = 2.782 -------------------------.3353

1/8 = 3.1297 -----------------------.3772

1/6 = 4.173 -------------------------.502

These are formulas for inch gear measurements-formulas for metric gears are different. I would strongly recommend purchase of a book on gear cutting so that you have a better understanding of the subject as well as having the formulas for reference.

Some help from a machinist friend, or a shop willing to lend a hand with formulas may be your best bet if you have difficulty understanding methods used in this article. If you are planning to do exact scale be sure to use accurate reference material because there are differences in the amount of teeth and pitch diameters on earlier vehicles of the same type. Foreign style tracks in which the tooth of the sprocket engages in the track block, and not near the pins, the same rule of spacing applies for pins and these holes also.

How to make a drill jig

Having the dimensions you can now fabricate a plate with a center hole. (Always use a center drill first to give your drill a pilot hole to follow and keep things accurate.)

{After locating first hole center go and bolt or clamp jig base to drill press. Drill first hole (after locating), unclamp sprocket blank, rotate and locate second hole, clamp and drill. Insert locating pin and drill hole through first hole into jig base-continue drilling.

Measure with your vernier caliper half the distance of the P.D. for the center of the hole by either inserting a drill shank or a pin into it and subtracting the half its diameter.

Scribe a mark for your next hole. The diameter of your next hole will be determined by the size of the horn link end connector or track block pin to be driven. After completing this hole measure in the same way from the center out to the P.D. Be sure to space over the C.T. (or pin space dimension). Double check your work when finished as it will be easy for things to be off. This then is your simple-as-can-get jig. You will have to do some further fabrication to elevate this jig in such a fashion as to allow you to rotate the subject sprocket after you drill each hole.

After you drill the first hole move it to the second where you will drop in a locating pin (which should be a tight fit) into a plate attached rigidly to the jig. The jig it self should be clamped tightly to the drill press table. With some care and a little luck you eill soon have a blank with holes accurately spaced.

I will leave the method of cutting teeth up to the individual because of the different shapes and types. I do recommend using jigs as they save time, and allow you to reproduce repetitive parts with greater accuracy.

The best and most accurate way to produce the sprocket jig and /or the sprocket hole patterns would be on a rotary indexing table. If you can find one to use- do it.

The previous method is meant to help those without one, and without the desire to spend the pile of cash it takes to buy one. If you have any questions about the above methods, formulas, etc. Feel free to write me at 19116 Schuster Ave. Castro Valley, Calf. 94546

I hope this has helped some of you become real treadheads!

Dave Spring:

I would to once again thank Dave Spring and Howard Thompson for their valuable contributions to this article. Although this article concerns itself primary with the sprocket on a Sherman, the basic construction information can be used with any sprocket design.

ELF1564@netmcr.com

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