Question pertaining to pitch diameter and teeth on timing pulley 1

[htngwilliam]

Guys

I need some help over here. I have a timing pulley system driven by a timing belt with 2 pulleys with a teeth ratio of 2: 1 [Large pulley = 38T, small pulley = 19T].

I wonder whether in timing belt design, the ratio is dependent on the number of teeth on both pulley or the PD of both pulley.

For example, if the large pulley is 38t and the smaller one is 19t, the ratio is 2:1

However, the pitch diameter of both pulleys may not be exactly 2:1 based on the tolerance, does the pitch diameter or other dimensions affects the pulley ratio?

 

[waross]

As long as the tolerances are not so bad that the belt starts jumping teeth, use the tooth ratio. If the belt starts jumping teeth it is either too loose or not matched to the belt.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MikeHalloran]

A timing belt pulley doesn't have an easily measurable feature that correlates directly to the pitch diameter.
The pitch surface is not within the pulley's envelope; it's at the effective center of the cord ply, somewhere within the belt.



Mike Halloran
Pembroke Pines, FL, USA

 

[htngwilliam]

Thanks guys.

I did an experiment to run the small pulley 40 times at 720deg interval then measure the output angle with a rotary encoder on the big pulley. My observation: -

Rev1) Small pulley = 720deg [Input], big pulley = 360.01deg
Rev2) Small pulley = 720 x 2deg [Input], big pulley = 360.02deg.

It seems like there are some cummulative error.

In theory, if the ratio is 2: 1, we suppose to get 360 deg each time.

 

[MikeHalloran]

Just out of morbid curiosity, what's the resolution of your encoder?



Mike Halloran
Pembroke Pines, FL, USA

 

[htngwilliam]

The resolution is 0.001

 

[MikeHalloran]

Uh, 0.001 ... what?

Revolutions?
Degrees?
Radians?



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

For synchronous belts the tooth ratio determines the ratio. The same number of belt teeth must pass over each pulley, just like roller chain. There are error sources within one pulley rotation. But over many rotations the ratio is exact.

 

[htngwilliam]

Hi

Sorry, I am using an incremental rotary encoder with a resolution of 0.001 deg.

I am using a timing belt system.

 

[kingnero]

Are you sure of the input (exact 720 degrees) as well?

 

[htngwilliam]

Hi

That is based on firmware control

 

[kingnero]

I wouldn't really worry about the .01 deviation, as said before I'm sure over many rotations the measuring errors will cancel each other out.

As long as the teeth don't skip, it can't go wrong...

 

[MikeHalloran]

Where did you get an encoder with 360000 cpr?



Mike Halloran
Pembroke Pines, FL, USA

 

[dinjin]

I would get another encoder.
The difference might have been backlash in the system not being tight when you started or you have gross errors on the flanks or loose centerlines.

 

[htngwilliam]

Sorry, my mistake. The encoder resolution is 0.01 not 0.001.

For timing belts like Gates GT3 series, I realise there is some wear/ drift after running the reliability test. I wonder how durable are these belts as I cannot find any information pertaining to belt life or wear.