Design of coupling

I think I already work for you. Don't change anything, but fix the problem. don't hesitate find a better coupling. the choice you make now will be with you as long as the equipment is. the money you spend on a coupling will be off set with reduced downtime & repair labor. you could just move the motor back until the pins can't hit the gear box any more, assuming the coupling doesn't fall apart or self destruct @ that point.

I am Sankar Narayan, was a rotating equipment specialist and now a consultant.
You may follow the following procedure for HT motor with high axial float:
• The OEM would have marked the 'rotor running center, for such motors with high floats. This mark can be seen on the motor shaft after the in-board bearing cover. If such mark is not fount, establish the 'running centre'.
• To establish and ensure 'running center', take the motor for a solo run. During this test run, ensure 'gear box (GB)' is well away from the motor coupling. This is very important to avoid the motor shaft/coupling rubbing/grinding the stationary GB shaft and causing, possibly, an accident. For this purpose, remove the 'holding down bolts' of GB and establish a gap of 50 mm between the couplings. Clear/clean the excess grease/oil sticking around the exposed part of the shaft.
• When motor runs on solo the shaft will jogs to the extremes for a while and finally runs at it its 'magnetic centre'. Using a sharp edge firmly against the running shaft, mark this position. This has to done carefully understanding/observing the direction of rotation, the free space at this area such that no injury is caused during this procedure. If preferred an experienced lathe turner could be availed for this intricate job.
• Coupled with load motor should run at this magnetic centre. If forced to run off this centre, excess power consumption, vibration, undue loading on the driven equipment axial bearing might exhibit.
• You have indicated that the axial float of the motor is 18 mm and the 'pin and bush' coupling is used. Pin and bush coupling can mange reasonable amount of axial play on run. Keeping the motor shaft at its running center, position and fix the GB such that the coupling gap (DBSE) is about 3 mm.
• Ensure this 3 mm gap after the alignment.
• While running, motor shaft will jog 3 mm (or more) back and forth and settle at 'running centre'. Motor coupling may touch the GB shaft for few times before settling at the magnetic center. That should be alright as long as it gets back to running centre sooner than later.
• I have seen some manufacturer employ a thin fiber, nylon, leather kind of flexible piece between the couplings as to absorb the hits during the startup.
• While running, the motor shaft should run at its magnetic centre and the gap has to be ensured around 3 mm. If this is hard to observe, help of a 'stroboscopic light' could be availed. Shaft heavily jogs, all the time while running, could be an indication of defective field in the motor.
Hope above procedure helps to some extend. Should you need more clarifications, please contact me at contact@yesyen.com or sankar@yesyen.com.
Sankar Narayan.
www.yesyen.com.

I think what u r thinking thats right. We had observed the Magnetic center on the motor. It was Littlebit out. Now wht we had did thats we had fix the gap 7.5 mm betwwen both the coupling & inserted a Teflon Bush of the same thk. at both the side. So it will restrict the movement. I just wanted to know that fixing of teflon bush with coupling, wht will be the effect on motor bearing, Bearing friction.pls reply on kirandpatel@rediffmail.com or can call me on +91 9879101923

I need more info on the following as to comment on fixing with two 7.5mm bushes:

• What is the coupling type, pin&bush or gear or grid or?
• What is the driven equipment's thrust bearing?
• How much is the float of this driven equipment (Gearbox)?

Keeping the above points in mind, the final position of this pair while running should be such that:

• Motor runs at its magnetic center.
• Gearbox input shaft runs on it active side of thrust collar (if tilting pad type).
• While so the DBSC is still sufficient, say 2/3 mm, as to accommodate the thermal expansions of both driver and driven shafts, which might grow towards each other.

Filling up the gap completely should not restrict the thermal growth.

If you want to discuss more on this, please mail me to: contact@yesyen.com or sankar@yesyen.com.

Sankar Narayan,

www.yesyen.com.