Timing belts

[CAM_MAN]

Does anybody have a preference on timing belt materials. I have snapped two belts already on my CADCUT on the X axis because the gantry is so heavy. They were urethane with kevlar cord reinforcement. I just purchased two more that are neoprene with fiberglass cord reinforcement hopefully they will work better. I have my accceleration turned way down in Mach3 to help abrupt stops and I am careful not to get the belt too tight or too loose.
And rather than start a new thread can anyone tell me how to set the clearance on a pinion gear and rack. I have just been bumping my rack to were my pinion has just enough room too move. It would be nice if there was a right way to do it.

[Carel]

Can you make a double-width pulley out of two, width only the outer flanges remaining and the grooves neatly lined up? Then you can use 2 belts running side by side.

Carel

[keyne]

How do you measure timing belt tension?
Do you have twist in the belt maybe?
At which point does the belt snap? Near the clamp?
What bending radius do you have in your setup?

I'm preparing for a belt drive myself, gantry weight maybe 50kg. Planning to use 1" HTD-5M.

[Carel]

It's all calculatable (is this English?) from the maximum allowed stress per 10mm for the different flavours:

T2.5 : 120N (=~12kg)
T5 : 330N
T10 : 780N

AT5 : 700N
AT10 : 1300N

HTD 3M : 100N
HTD 5M : 208N
HTD 8M : 375N
HTD 14M : 425N

MXL : 39N
XL : 56N
L : 98N
H : 235N

So for more load you can go up in width in the same size, increase the pulley to decrease stress, or go up in size. Another rule is a minimum contact of 6 tooth on the smallest pulley. For example: this rules out a 12 tooth pulley for a reduction.

Carel

[mxtras]

Originally Posted by CAM_MAN ...And rather than start a new thread can anyone tell me how to set the clearance on a pinion gear and rack. I have just been bumping my rack to were my pinion has just enough room too move. It would be nice if there was a right way to do it.

Most of the commercial machines I have seen or serviced set the pinion on a pivot and keep it engaged with a relatively stiff spring. I have seen tension springs on most machines and on higher end machines I have seen bellville washer stacks used to keep the pinion in mesh. There is no gear clearance other than the minimal amount created by the scarse lubrication (grease).

Scott

[CAM_MAN]

Thanks for the info on the belts. I will deal with it til my next two belts break and during that time I will look into going to a wider belt and pulleys . It is irritating that I bought plans for a machine that consistently gives me grief. I should have known better than to have believed a 3/8" .200 pitch belt was going to last very long throwing a couple hundred pounds back and forth. Also thanks for the info on the gear rack. I am probably going to eventually redesign my gantry to run off two motors and eliminate the driveshaft, pulleys and belts altogether. That way I can spring load the motors to keep even pressure on the pinion to rack. Thanks guys!

[2muchstuff]

You can get an estimate of the forces present when reversing by taking the gantry weight and figuring backdriving force at speed, multiplying that by the ratio and adding the motor torque.

[energyforce]

What is your max acceleration and gantry weight?

[CAM_MAN]

Originally Posted by energyforce What is your max acceleration and gantry weight?

Gantry weight is probably about 150lbs. I have my acceleration set at 10 in Mach3.

[ger21]

What type (profile) of belts are they? I believe GT2's are much stronger than most others, based on what I've read from SDP-SI.

[energyforce]

I dont know what 10 means in mach 3? 10in/sec^2? Ive never used mach, at least not for another month.

I think they are trapezoidal belts if thats what they are called. I love brecoflex's site. Its totally automated, with all the details listed as you go (i.e. minmimum pulley diameter, pulley spacing etc).

If you go to www.brecoflex.com and select belts you can read about mine. Mine is the AT-5 25mm. I know you really dont wanna go with a wider belt cuz thats just too expensive eh but I would recommend maybe calling brecoflex, talking to their engineer and tell him which belt you have and what replacement belt he has. Check and make sure its a stronger one? Breco has a LOT of different belts with a LOT of different materials and cording.

Cam-Man I will have some pics up on monday and even more next weekend and you can see what im doing?

[CAM_MAN]

Originally Posted by ger21 What type (profile) of belts are they? I believe GT2's are much stronger than most others, based on what I've read from SDP-SI.

The one giving me fits is a 176XL. It is 3/8" wide .200 pitch trapezoidal belt that is 17.6" long.