Re: Timing Belt styles/sizes

Chris:

I went through these thoughts specing a retrofit for my mill. From this and
other applications it seems the round tooth profiles (HTD & GT) carry more
power (torque) for the size than the trapaxoid profiles (XL,L...). For my
mill I used HTDs. They seem very similar to the Gates GT form but are not
proprietary. I wanted to stay away from proprietary belts so I could replace
them from anyone. The pulleys and belts I purchased were Gates though.

I had not heard that the tarpazoid form had more backlash than other forms. I
find it hard to believe a timing belt will have any backlash. One form, due
to tooth shape and flex may give a cyclic change in position with each tooth,
like cycloidal action on a chain sprocket. I suspect this will be negligible
though, but Gates knows a lot more about belts than I. I would be surprised
if this makes any difference on a hobby class machines. (i.e. one without
ground ball screws in a heck of a rigid machine)

My mill uses steppers rates at some 750 oz-in. I don't think this relates
directly to your synchronous motor but will give a point of reference. I used
5mm HTD belts and sprockets. The X and Y were, I'm sure, oversized since I
had the room. My thought was larger pulleys and wider belts would reduct
stretch and increase accuracy. But the Z-axis on the quill is tight and I
finally decided to use 5mm HTD belts 15mm wide with 18 teeth on the motor to
36 teeth on the ballscrew. I have not hooked it up but I think it is in the
ball park for this set up.

The trick is estimating the loads the belt will see. A stepper will see
considerably less than its holding torque. If it sees its holding or max
torque it is likely to lose steps. How far to be away from this and what cut
gets you to this level is the tricky part.

I hope this helps some. If I have miss-stated any of thisI hope others will
jump in and correct me. Thank you.

On Wednesday 16 January 2002 08:40 am, you wrote:
> While trying to pick a timing belt style for the long feed leadscrew
> drive on my lathe, I downloaded a manual from Gates.
>
> In this, they mentioned that the HTD style of belt is designed to
> be stronger, but the tooth shape used to accomplish this requires
> greater clearence than trapezoidal belts, which results in backlash.
> In short, they do not recommend HTD belts for precision positioning.
>
> I had the sense that some here did like these belts for this purpose.
> Gates may have a vested interest - they are trying to sell their
> new shape, the GT2, but that does not necessarily void their argument.
>
> What belt series in practice is good for a 600 oz-in motor? A 1000
> oz-in? I used XL on the crossfeed stepper, but assume I need
> something bigger such as H or L for the long feed given that I'm using
> servo with much more power. I've been able to find a few figures for
> allowable working tension, but it is unclear if that is installation
> pre-tension, or the tension due to power transmission.
>
> Chris Stratton

--
Hugh Currin, PE
Mechanical Engineering Technology
Oregon Institute of Technology