Help with a backup manual drive mechanism for a chain drive system

[JDCoyle]

Good afternoon, and thank in advance for any help or suggestions. Apologies if this is not posted in the correct area!

I have a chain driven mechanism that raises and lowers a sensor, and I have previously designed a manual drive back up for the emergency case where we lose power to the drive motor (or the motor fails). This is to prevent damage to the sensor. Everything worked well until the client changed the sensor to one much heavier than the previous. The motor that we were previously using now back drives (lowers the sensor) under just the hanging load of the new sensor. We then upgraded the motor to a 90 degree drive motor which cannot be back driven (at least not at this weight). Unfortunately the new motor cannot be manual driven either!

So I want to introduce a method of disengaging the motor from the chain drive system.

I have attached a rough sketch of my concept in doing this, but I feel as though I am missing a simpler solution to the problem. I'm the only engineer on staff here, so I have no one to bounce ideas off of, so looking for some back and forth discussion. The chain drive sprockets (shown in red) are the only thing that is fixed at this time. They cannot be moved from their current location, all other components are up for debate.

 

[btrueblood]

If the sensor is that heavy, what keeps the gear teeth from getting worn prematurely as they start to slip past each other when "just disengaged". I'd lean more towards a manually actuated clutch, so you could let the clutch slip a bit to allow the sensor to fall in a somewhat controlled fashion.

 

[gruntguru]

Off-the-shelf solutions are always to be preferred.

Dog clutch? If you don't need the gearset, a dog clutch (very common in marine drives) would allow direct drive.

je suis charlie

 

[tbuelna]

Is there an anti-backdrive capability required on the manual drive system independent of that on the motor drive system? Or is the motor drive system just re-engaged while force is being applied to the manual drive?

 

[JDCoyle]

Thanks for all the replies!

COTS solutions are always best, would love to find something to just drop in.

So the "safe" position for the sensor is in the up or fully retracted position. I have a set of positive locking pins for when the sensor reaches this position, which engage mechanically. So in the instance where we lose power or the motor, we manually drive the sensor back up until the pins engage. Then we have no issue with back driving.

 

[BrianE22]

Make sure you look at what happens on the load transfer from the self locking shaft to the manually driven shaft. I'd be concerned that if you're not quite ready to apply torque with the manual system, the load will fall during the transition. Maybe a friction clutch where you can gradually control the transfer of load would be a better choice. How did the brake work in the original system (before self-locking)?

 

[JDCoyle]

The current design does not have a brake. There are limit switches at the full extend and full retract positions. At full retract the positive lock mechanism is engaged, and at full extend the sensor mount plate rests on dense rubber pads.

 

[racookpe1978]

Shock and vibration limits and tolerances? (Your title is aerospace, will this gadget ever be inverted or subjected to vibration loads?)