LONG Timing Belt application question 3

[tygerdawg]

I am designing a single axis linear drive for a client. For several important design reasons I wish to use timing belts if possible. My fallback position will be rack & pinion, but that causes some other problems.

It's a strange application: 57 ft pulley-to-pulley distance, open-ended belt clamped to a carriage in linear bearings, carriage attached to a 6500 lbs buggy on flanged casters, reciprocating back/forth, +/-0.25in position tolerace, slow speeds and accels. I'll drive it with as big a gearmotor as needed.

I've done an exhaustive analysis of torques, inertia, horsepower, tension loads, etc. Several buggy sizes will be towed by the carriage, so I have a good understanding of the range of torques required and have made system assumptions and adjustments to make this work.

Now I'm at the point of selecting a belt and have hit a brick wall. I've contacted Gates, Goodyear, Carlisle, and Mectrol and have gotten nowhere with these guys except for Mectrol. All except Mectrol ignore me because in their words "no one knows how to calculate the tensile limits on slow moving belts". It seems their entire market is constantly moving endless belts like on your car and my app is so oddball that no one wants to touch it. What I have done is iteratively plug my numbers into each company's design calculator and generate strength, rating values, and recommended part numbers as if my app were endless belts of varying length. From this calculated data and their catalogs, I try to infer working tensile loads, tooth shear strengths, and horsepower capacity from the results. All the sizing exercises in the catalogs are very simplistic and are based on RPM/Torque/HP/length/pitch and constantly rotating systems. The results from all vendor sizing calculators are encouraging, but inconsistent enough with my own calculations to cause me concern. I've narrowed my choice to a certain pitch, tooth style, material, and width. But without explicit confirmation, I'm leary.

My gut tells me that if I put a big enough honkin' belt on this system, it will pull the load. But no vendor will commit to an answer, either "use this one" OR "our belts won't work". And I'm tired of teaching mechanical engineering to application engineers.

Can anybody out there provide any insight?

Thanks in advance

TygerDawg

 

[btrueblood]

I can't see any reason not to use roller chain or cable drive, with motor/encoder at each end, along with a (weak) brake. Pull with one motor, keeping brake drag on opposite side to maintain chain/cable tension. Reverse operation (pull with motor #2, brake at motor #1) to reverse direction.

I'd worry a bit with cables - they do permanently stretch/creep over time and/or load cycles. You want to operate well below their breaking strength (typ. load factor of 4 or 5), and apply a "proof" load to pre-stretch them before final installation. We use a cable drive for one of our valves here (not my idea), and it's a pain in the butt to pre-stretch the cable assemblies and then take up the slack that is generated. No matter what I've tried, there's always a bit of "slop" in the system... but we are worrying about a few hundredths of an inch of slop in 24" of cable, not 3/4" in 20 ft, I calculate we are looking for about an order of magnitude more precision than you are.

 

[Barry1961]

I have a few machines using Gates PolyChain that have over 20' center distance on pulleys and have a high cycle rate. The last time I saw one was a couple of years back and the belt had not strecthed at all and it had been in operation over 8 years. They are stacking 500lb steel rolls at 60 in-sec with about 80 in-sec^2 accel, 2 shifts.

We have a welding machine moving a 2500lb bridge that would repeat within .010" in a 18' travel. If you pushed the bridge you could move it probably .060" but it would return to the original spot if commanded to move away then back. If this is not good enough you take about .010" off the OD of sprocket to tighten it up or Gates now has slightly oversized sprockets for low slop applications.

I would not use the 20MM pitch belt at all for your application, the 14MM pitch is much better. The teeth on the 20MM will cog over the sprocket to easy for E-Stops.

When we got over about 12' we would use a little UHMW wear strip to support the belt. If you don't the required belt tennsion gets high and you start getting vibration/bounce in the belt/system.

I have used a bunch of PolyChain in some very demanding dirty applications and it has never failed. Dayton/Browning came out with something very similar a couple of years back called Panther that will run in Gates sprockets in low speed applications after a minor wear in.

I hate to sound like a Gates salesman but the PolyChain is an outstanding product. It was developed for the belt drive on the Harley and I think is now used in the Northstar engine. It does not strecth or wear like roller chain.

Barry1961

 

[tygerdawg]

Thanks Barry. I finally got a response from a couple of vendors, cleaned up my calculations and other assumptions, it looks like we can make it work with belts. Thanks for the tip about belt support, I was planning on something but it's good to have it confirmed.

TygerDawg