It all started with a $ 75 jaw price reduction for one dental pulley for a special drive system.
The application is simple, the drive system requires positive traction with minimal side loading. The toothed belt provides the ability to transfer loads like a chain, but better. However, the costs of entering this solution are high, mainly because the typical gear pulley is made of steel billet using the tamping method. Secondly, the costs are high because the demand or availability for such a gear pulley is low.
No one wants to enter the market where a special pulley is needed. A V-belt pulley is available, but has a downside in which it transmits friction for transferring the load from the belt to the pulley. The frictional load on the belt must increase in tension as the load and revolutions on the pulley increase, which leads to heating of the belt, weakening the tension, as it expands and causes a vicious circle of slip and loss of power.
To achieve the correct transmission of horsepower requires more belts, which complicates one time simple design.
However, the timing belt is simple in design. The toothed pulley effectively and calmly transfers the load without the high tension needed to transfer the load. The belt acts more like a chain, but is lighter and in some cases stronger.
The experiment is as follows: use a three-dimensional printer to develop a special gear pulley. Use this 3D printed piece as a pattern. Then use the highest durable urethane material to release a number of special toothed pulleys. These gear pulleys can be subjected to a drive system to see what they are doing.
Preliminary calculations show that the stresses for the pulleys must be below 7000 psi of material stress levels, which can lead to a failure. For example, the belt is designed to emit about 120 pounds in the worst case for loading. This force will be transferred to the torsional load, which imposes a load on the keyway. The highest load on the key deck was about 3000 pounds per square inch.
However, the load on the toothed pulley is complex and can be summarized in the following variables:
- Preload.
- Tension load during operation.
- Loading tape.
-The speed and temperature change due to the frictional load on each tooth.
- Centrifugal loading.
-Shock loading due to acceleration and deceleration.
The hardest thing to predict the thermal load on each tooth. Thus, at the moment the question arises: how long will the urethane last? What will be the failure mode?
This is an experiment.
Why bother? Because we can learn from this experiment and sometimes come up with another solution, more cost-effective. Another note - the possibility of the drive and the ability to solve this problem with a simple toothed belt drive will help to complete the solution without the use of V-belts with high stretch and weakened power.
Urethane Auger Test Results 8-22-14:
Uretian pulley using Task 2 for Smooth-On failed after about 10 minutes of use. The failure was due to friction caused by heating on each tooth, which caused the teeth on the drive pulley to be cut.
The Task 2 material, although strong enough to withstand the load, especially when it is cold, tested at full capacity without any side effects (for example, cutting teeth due to too high belt tension), could not work with a long service life because that the heat index of the material is about 150 - 160 F.
Other material with a higher heat index will be investigated as of 8-29-14.
The overall drive system using a gear pulley was easily tuned and worked perfectly when transferring the load from a 10 hp engine. on the drive. A full 10 horsepower was carried forward, which was a good result.
Test Results II Completed 9-10-14
Epoxy resin was used to cast two more pulleys. Epoxy resin 670 HT has a thermal conductivity limit of 252 F (350F when cured) without curing. The material has a hardness of 90 D, a limiting tensile of 4500 psi.
The initial assembly and preliminary test showed minimal belt heat with good traceability and minimal sprocket wear, if any. Weather conditions did not allow us to carry out these tests.
Epoxy Results
The test was interrupted prematurely because the belt broke. It was assumed that the belt broke, because it was undersized. But after a sequential calculation, it was found that it is within the normal range. This problem arose in consultation with the manufacturer of the belt. In a particular project, a twisted belt drive system was used, which by its nature puts unwanted lateral load on the timing belts during the cycle cycle.
The toothed belt is designed by nature to transfer loads with a uniform tension on the cross-section of the belt. When the belt is twisted, then one side of the belt receives the entire load, and the opposite side receives compression. Very similar to the beam when loading. (Example: a person standing on a diving board.) The upper part of the beam receives tension, the lower part of the beam receives compression. When the beam fails, the upper part usually cracks first, and the bottom usually remains untouched.
Toothed belt goes the same way. One side will alternate in loading, loading, and then unloading with maximum system tension. As a result, the tape will be loaded with a maximum tension on a part of the cross-sectional area of the belts or on a part of the design load. The belt did not succeed immediately, when we put a full load on it, probably essentially prefixes from previous other disks.
Testing has shown that our drive system is unacceptable, but the drive pulleys were maintained fairly well.
When disassembling the pulleys were inspected and showed cracking. The material, although it was tough and capable of withstanding high temperatures, was brittle, and the shock load of the keyway besieged failure.
A pulley malfunction was not in its durability, but in its ability to shock. Future designs will be to wrap the shaft-keyed sections of the fiberglass mat to increase overall strength.
As a recommendation for a urethane epoxy pulley, the results are positive. In the future, we can explore the toothed pulley on a wooden map, using toothed driving pulleys and composite pulley wheels, but this is for another day.
