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[Frank DeWitt]

Making the pulley with one flange allows them to use a simple two part mold. The mold costs less and the parts are made faster then if it had two flanges.  It isn't good design but it does save money. 

(I am a mechanical designer, I use timing belts often, and I have designed plastic parts and there molds. I have never seen this done.)

[Bob Leonard]

Having been an experimental machinist/model maker for about 20 years in my previous life I'll state for a fact that you could certainly produce the same part with two flanges using a 2 piece mold. Poor design is the only answer here. If I owned an X32 I would be mighty tempted to remove every sprocket and either Loctite it back on, or put a 2-56 set screw in every one of them.

[Steve M Smith]

Bob is right.  It is common practice to have a flange both sides on drive belts.



The Behringer design appears to use the surface of whatever is below the gear to retain the belt.  This will probably work 99.99% of the time but if the gear is only a press fit on the shaft, a small quantity are going to move upwards at some point and cause a problem.


Steve.

[John Roberts {JR}]

 I am not keen on second guessing their design. It seems a liquid adhesive could secure the pulley to the motor shaft. If they need to tool a new pulley they can probably afford to do that now.

They seem to have had pretty good success so far.

JR

[Bob Leonard]

Maybe John, but if you notice that the sprocket Steve has shown has a hub and both flanges. The hub is the ideal spot for a set screw which eliminates the movement on the shaft while the flanges retain the belt. A quick look at the Behringer design and it's apparent that height is a factor. The motor shaft isn't long enough to protrude through a redesigned sprocket with a hub and two flanges, and height above the sprocket may be constrictive.

The other problem is this. The design and mold process has probably resulted in a slight taper of the sprocket surface, maybe as little as .001 - .002 to allow for quick removal from the mold. That taper will eventually result in the belt forcing the sprocket up allowing the belt to slip underneath as in the example. It may be that Behringer's design could be modified by placing a set screw as I mentioned above, in the low point between the sprockets teeth where it will not effect movement or create excessive wear on the belt. 

[John Roberts {JR}]

Maybe John, but if you notice that the sprocket Steve has shown has a hub and both flanges. The hub is the ideal spot for a set screw which eliminates the movement on the shaft while the flanges retain the belt. A quick look at the Behringer design and it's apparent that height is a factor. The motor shaft isn't long enough to protrude through a redesigned sprocket with a hub and two flanges, and height above the sprocket may be constrictive.

If the clearance above is that tight, that should keep the sprocket down. 

The other problem is this. The design and mold process has probably resulted in a slight taper of the sprocket surface, maybe as little as .001 - .002 to allow for quick removal from the mold. That taper will eventually result in the belt forcing the sprocket up allowing the belt to slip underneath as in the example. It may be that Behringer's design could be modified by placing a set screw as I mentioned above, in the low point between the sprockets teeth where it will not effect movement or create excessive wear on the belt.

Using set screws, in a mass production environment, on a high volume product is undesirable added labor (and even the cost of the screw, not to mention tapping the hole the set screw screws into, etc). Some hard setting adhesive seems appropriate if this is a real problem.   

I am surely repeating myself but this is a deep value product (good for the money).


JR

[Steve M Smith]

I am surely repeating myself but this is a deep value product (good for the money).

Despite some armchair redesigning with the benefit of hindsight, I agree with you.  Behringer are to be congratulated on the X32 which is really quite an achievement.

The problem of the belt slipping off the pulley is probably very rare and for all we know, they might have already addressed this problem themselves.


Steve.

[Andrew Broughton]

With 3d printers commonplace now, I'm sure someone could create a new pully if it becomes a problem. It would be an interesting experiment.

[Frank DeWitt]

Having been an experimental machinist/model maker for about 20 years in my previous life I'll state for a fact that you could certainly produce the same part with two flanges using a 2 piece mold.

I don't see how this could be done.  I could see it with a flanged pulley without teeth but not for a timing belt pulley. 

To be clear.  I agree, timing belt pulleys should and usually do have two flanges, and I think they should.  I am only saying that the mold is a bit simpler if you cheat and make it with one flange.  Not a good design.

 

[John Roberts {JR}]

I continue to be impressed with the depth of experience in this forum. I myself have only designed one injection molded part (the plastic case for my current product), so I have a rudimentary, but not expert understanding of draft angles, and sundry IM design rules.

Uli needs to ask you guys how to do it next time. :-)

JR

[edit] are any of you guys experts about linker scripts for my embedded DSP debugging environment? I am having trouble with variable visibility, and suspect it is related to the linker. [/edit]

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