Emergency Drive

Why not just a hydraulic cylinder?

the stroke requirement is 6100 mm. Quite long. the rack and pinion are in mesh at all times. the torque requirement at the pinion is huge. the gear box has a reduction ratio of 1:80 and so it is irreversible, as per the design intent. the emergency drive is required to be placed elsewhere, but can utilize the same rack and separation of primary drive is required before getting the secondary one in place and all these shall happen remotely.

Normally, shaft couplings are reliable devices. Has the root cause of the failure been determined? Was the coupling designed/selected to handle the maximum torque of the motor? Maybe you could install a back-up motor on the rack. Or use a hydraulic cylinder.

it is true that the couplings are mostly reliable. in the true sense whatever happened with us was not the mechanical failure of the top coupling shown in the drawing but there was drift of the coupling towards the gear box resulting in losing contact with the output shaft of motor. the driving motor was unable to transfer rotary movement to the input shaft of the gear box and the drive was rendered useless.

adding a snap ring

You said that it was your coupling which failed, so probably a redundant drive system would be useless if you have a broken coupling, unless you have a double ended input shaft to your gearbox, that way you could drive both ends with a coupling each; that would reduce (by a whole lot) the stress at the coupling connection.

that is true. our gear box on one side coupled to the input shaft of the motor, on second it is connected to a brake, on third the pinion shaft and on the fourth one it is connected to the control unit that gives the online reading of the distance traveled by it. having one more driving element is quite difficult.

Motor couplings are pretty reliable.

Best you describe why or the reasons that the area is inaccessible. As that would aid in determining a back up system.

Generally most these couplings don't take long to replace if the parts are on hand. So what do you mean by good amount of time?

it is used with a nuclear reactor fuel charging mechanism for loading uranium bundles into calandria, so normally at all times it is in the inaccessible zone. these mechanisms only become accessible during annual planned shutdown or accidents necessitating maintenance of it after unplanned shutdown of the reactor.

In order to eliminate all potential failures for all components you would need a duplicate of force provided by this system and an override of the failed system. If it is a one-shot emergency system, maybe a driven cable or chain arrangement.

Do you have a frequency of failure issue or estimate of time frame? Should you configure a turret of replacement couplings\

(Knowing nothing of such machinery...)

Has an effective fault alerting system and/or failsafe mechanism been considered?

Even 6 m is not unusual for a hydraulic cylinder (you can also use flyer chains or cables to reduce the stroke of the cylinder though you'd add mechanical parts).

Overall for linear motion hydraulic cylinders are much more reliable than other solutions. Unless a good reasons probibits hydraulics I wouldn't mess with mechanical solutions (I remember the discussion about small hydro Pelton injectors driven by crappy DC motors).

I am herewith sending the arrangement of the drive for better perception. please look into it.