The 3D Printer, The Urethane Pulley and The Go
Kart Experiment
It all started with a jaw dropping price of $ 75 for one tooth pulley for a special drive system.
The application is simple, a drive system requires positive traction with minimal side loading. A toothed belt provides the ability to transfer loads much like a chain, but better. The costs for entry into this solution however are high mainly because the typical cogged belt pulley is manufactured out of a steel blank using the hobbing method. Secondly, the costs are high because the demand or availability for such a cogged pulley is low.
Nobody wants to enter the market, where a special pulley is needed. A v-belt pulley is readily available but has a down side in that it relies on friction for its load transfer from belt to pulley. The frictional load on the belt has to increase in tension as the load and rpm increase on the pulley, causing the belt to heat up, loose tension because it has expanded and cause a vicious circle of slippage and power loss.
To attain the proper horsepower transfer, more belts are needed, which complicates the once simple design.
The cogged belt however keeps the design simple. The toothed pulley transfers the load efficiently and quietly without the high pretension required for load transfer. The belt acts more like a chain, but is lighter and in some cases stronger.
The experiment is this: use a 3-d printer to develop a custom cogged pulley. Use this 3-d printed part as a pattern. Then use the highest strength possible urethane material to cast a series of custom cogged pulleys. These cogged pulleys can then be subject to a drive system to see what they do.
Preliminary calculations show that the stresses for the pulleys should be underneath the material stress levels of 7000 psi that would cause failure. For example, the belt is calculated to put out around 120 lbs at the worst case scenario for loading. This force would be transferred into torsional loading, which would put load on the keyway. The highest stress loading on the keyway was around 3000 psi.
However, the loading on a cogged pulley is complicated and can be summed up in the following variables:
-Tensional Pre-load.
-Tensional Load during operation.
-Tooth-Belt loading.
-Speed And temperature variation due to frictional loading on each tooth.
-Centrifugal Loading.
-Shock Loading due to acceleration and deceleration.
The hardest to predict is the heat loading on each tooth. So the question at this point is: How long will a urethane part last? What will the mode of failure be?
That is the experiment.
Why go through the hassle? Because we can learn from this experiment and perhaps come up with a different solution that is more cost effective. Another thing of note is that the drive line is peculiar and being able to solve this problem with a simple cogged belt drive will help finalize the solution without using high tension v-belts and loose power.
Urethane Pulley Test Results Performed 8-22-14:
The Urethane Pulley using Smooth-On's Task 2 material failed after about 10 minutes of use. The failure occurred because of heat induced friction on each tooth, causing the teeth on the drive pulley to shear off.
The Task 2 material, though strong enough to handle the loading, especially when cool as testified by the full output of power with no adverse effects (such as shearing off teeth because of too high a belt tension), was not able to handle long term use because the heat index of the material is around 150 - 160 F.
A different material with a higher heat index is going to be investigated, as of 8-29-14.
The overall drive system using the tooth pulley was easily tuned and worked excellent at transferring the load from the 10 hp engine to the drive train. The full 10 hp was able to be transferred which was a good result.
Results Test II Performed 9-10-14
An epoxy resin was used to cast two more pulleys. The Epoxy-Cast 670 HT Resin has a heat limit of 252 F (350F if cured) without curing. The material has a hardness of 90 D, ultimate tensile of 4500 psi.
Initial assembly and pre-run test showed minimal belt heat up with good tracking and minimal sprocket wear if any. The weather prevented us from performing actual drive tests.
Epoxy Resin Results
The test was cut short prematurely because the belt broke. It was assumed that the belt broke because it was undersized. But after subsequent calculation inspection it was found to be well within design limits. A consultation with the belt manufacturer revealed the problem. The particular design utilized a twisted belt drive system which by nature puts unwanted side loading on cogged belts during the run cycle.
A cogged belt is designed by nature to transfer loads in an even uniform tension across the belt cross section. When the belt is put in a twist then one side of the belt gets all the load, while the opposite side gets compression. Much like a beam when it is loaded. (Example: person standing on a diving board.) The top of the beam gets the tension, the bottom of the beam gets compression. When a beam fails the top section usually cracks first while the bottom is usually intact.
The cogged belt fails similarly. The one side will alternate in loading, being loaded and then unloaded, with the maximum tension of the system. As a result, the belt will be loaded up with maximum tension on a fraction of the belts cross sectional area or a fraction of the design load. The belt failed instantly when we put full load on it, probably having been tensioned prematurely from the previous other drives.
The testing revealed that our drive system was not acceptable, however the drive pulleys held up pretty well.
At disassembly the pulleys were inspected and showed cracking. The material though tough and able to handle high temperatures was brittle and the shock loading of the keyway precipitated a failure.
The failure of the pulley was not in its durability, but in its shock loading ability. Future modifications to the design would be to wrap the shaft-keyway sections with fiberglass mat to enhance overall strength.
As a recommendation score for a urethane-epoxy pulley, the results are positive. We may explore the cogged pulley on a wood go kart in the future, utilizing cogged drive pulleys and composite wheel-pulleys, but that for another day.
Robert Gamble is an entrepreneurial engineer that dabbles in various applications from urethane to go-kart design. He is most known for his Go Kart Design books and programs that simplify go kart drive line design. In this application a peculiar cost saving design was being investigated and failed, however, this failure shows the low cost to failure, versus the high cost to find out the design was ill fitted.
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