Tip To Improve Slow Speed Performance Of Servo Motor With Reducer Pulley

[Wizcrafts]

I sew at work on a Cobra Class 4 heavy stitcher, which is equipped with a servo motor containing a digital speed readout and up/down speed buttons. Since the machine arrived in late-January I have struggled with trying to get it to sew steadily at very slow speeds, without dialing it way down to its minimum top speed (useless on long runs). Also, it was very difficult to turn the flywheel by hand, to single stitch around belt tips, or pre-position the needle before entering a previously made hole (doing repairs).

This morning I was talking to Steve, at Cobra, and told him about this problem. He suggested that I try loosening the belt going from the speed reducer to the machine and also the motor to reducer belt. The reducer is bolted to a slot in a side post on the frame and can be moved up and down, to tension the upper belt. The motor has a threaded rod with nuts above and below a tang on the motor. Raising the position of the tang loosens the motor belt and vice-versa. Both of the belts on our machine were very tight and had minimal deflection in their middle areas (under 3/8").

Today, we loosened both belts slightly (to ~1/2" light touch deflection) and I am here to tell you that it made a big improvement! It is now easier to obtain and maintain slow speeds (~1/second), before the motor drops out. Furthermore, it is also easier to hand wheel for single stitches, or to position the needle where I want it to line up. Mind you, the hand wheel is still loaded by the motor and nowhere near as easy to turn as a machine hooked up to a clutch motor.

This is a no cost solution for anybody who has a big sewing machine with a speed reducer between the motor and machine. It has benefits to the motor, reducer and machine, in that it reduces the strain on the bearings. You just have to make sure you don't loosen either belt so much that it slips when you sew dense or very thick leather. There is a sweet spot and you can find it with a little experimentation. If your work is always medium temper leather you can probably loosen the belts even more, before slippage occurs. A good belt dressing can give better grip, at less belt tension.

Happy sewing to all and to all a good night!

[Art]

How true. I have visited many machines where the belts are tensioned to play something close to a high C if plucked. As a lot of these were Cobra machines, I asked why the belts were so tight. Answer was, "they were pretty loose when delivered so I tightened them up". This does not work well with a bushing machine. The bushings are quite adequate for even continuous use, but require oil. When you set up ungodly tight belts, you somewhat compress the downside of the bearing and rob it, to some extent, of the oil film (which IS the actual bearing) that the shaft rides upon. In constant use this will cause bearing failure (usually unacceptable shaft slop) after time, for the leatherworker this may be years, but it just isn't good. If you have a machine that uses ball or roller bearings (usually a Pfaff), they are made to take side loading and the belts can be set up pretty tight.

As far as lower torque in a motor at slow speed, the speed reducer helps a lot. I considered producing a motor that had full torque regardless of speed, but the 1/2 to 3/4 HP motors and controllers I consider adequate for big stitchers would have a market price starting around $500 (US made), possibly more. The Chinese would knock it off in a heartbeat, so I'm not interested in doing free engineering for them. But it can, and has been done. This is why we don't see much in the way of improvements in a lot of industries. The Chinese are by and part NOT innovators, the Germans and Americans (and often the Italians) who can innovate, won't do it and just give it to the Chinese (through espionage or just plain theft). So innovation has to be amortized at an accelerated rate, as does marketing and manufacturing so profit can be made in those first few years before the knockoffs start. Low volume products will not even be attempted given that model.

Sorry soapbox off.

Art

I sew at work on a Cobra Class 4 heavy stitcher, which is equipped with a servo motor containing a digital speed readout and up/down speed buttons. Since the machine arrived in late-January I have struggled with trying to get it to sew steadily at very slow speeds, without dialing it way down to its minimum top speed (useless on long runs). Also, it was very difficult to turn the flywheel by hand, to single stitch around belt tips, or pre-position the needle before entering a previously made hole (doing repairs).

This morning I was talking to Steve, at Cobra, and told him about this problem. He suggested that I try loosening the belt going from the speed reducer to the machine and also the motor to reducer belt. The reducer is bolted to a slot in a side post on the frame and can be moved up and down, to tension the upper belt. The motor has a threaded rod with nuts above and below a tang on the motor. Raising the position of the tang loosens the motor belt and vice-versa. Both of the belts on our machine were very tight and had minimal deflection in their middle areas (under 3/8").

Today, we loosened both belts slightly (to ~1/2" light touch deflection) and I am here to tell you that it made a big improvement! It is now easier to obtain and maintain slow speeds (~1/second), before the motor drops out. Furthermore, it is also easier to hand wheel for single stitches, or to position the needle where I want it to line up. Mind you, the hand wheel is still loaded by the motor and nowhere near as easy to turn as a machine hooked up to a clutch motor.

This is a no cost solution for anybody who has a big sewing machine with a speed reducer between the motor and machine. It has benefits to the motor, reducer and machine, in that it reduces the strain on the bearings. You just have to make sure you don't loosen either belt so much that it slips when you sew dense or very thick leather. There is a sweet spot and you can find it with a little experimentation. If your work is always medium temper leather you can probably loosen the belts even more, before slippage occurs. A good belt dressing can give better grip, at less belt tension.

Happy sewing to all and to all a good night!