Parallel Shaft Gearbox With Two Motor Input

[AltaIron]

I'm looking into a Falk V Class parallel shaft gearbox. This will be driven by two inverter duty motors at 10 hp each. Would it be correct to size this gearbox for 20 hp times necessary service factor? Would the motors have to be speed matched to get the 20 hp? Seems like a simple question, but it's got me puzzled. If one motor was slightly slower than the other would it then actually provide power? See attached sketch for clarification.

 

[racookpe1978]

Yes, they must be speed matched.
Yes, it needs to size for the 20 hp. Not for drive itself, but because at some time or another, one of the motors will fail (or will be "behind" the first motor) and so the entire load will come up to the first motor for a short time. At that overload, the first motor (you hope!) will fail/drop out due to over-currrent, and then both loads will fail.

Under the real world, one or the other loads will vary up and down as things come up and vary: clumps, torques, seizures, feed rates change, humidity, pressure, temperature, etc. You've got a difficult problem. Can you use clutches on the two loads to drop off one or the other rather than kill both?

 

[AltaIron]

My "customer" has no clutches in the design and I'm not entire sure if they are planning on driving with both motors. They may have it set up to drive with one motor and have the other motor be a fail safe. I don't have any information on the required output power. They just gave me motor information and what to size the gearbox for which was 10hp input. Then I saw their drawing (not the sketch I posted) and it showed two 10hp motors on the gearbox. I know I need to get some clarification from them I just wanted to get some opinions before I approach them.

Thanks for the response.

 

[racookpe1978]

If there are no clutches, then they have no choice but to continuously both motors and both loads. A "dead-head" motor being driven backwards from the other motor is not recommended for any long-term operations.

 

[JohnRBaker]

If the second motor will truly be used ONLY as a failsafe, or back-up, and there is some way to assure that both motors will never be providing power at the same time, then it would would seem that this is only a 10-hp drive situation. It also means that the motors would not have to be speed matched.

As for clutches, if both motors were either equipped with mechanical one-way clutches, perhaps centrifugal, or electrically actuated so that you could wire them to be mutually exclusive (only one could be engaged at a time), then I think this would work fine using a gearbox rated at 10-hp and non-speed matched motors.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[BrianE22]

If you had VFDs controlling the motors you could run them in torque control. Then, at least for steady state, each motor could supply the same torque. The guys in the electrical forum on motors could you tell you more.

 

[JohnRBaker]

But if the two-motor configuration was truly intended as a means to provide for a 'fail-safe' situation, then worrying about torque- and/or speed-matching would just be a waste of time and resources. Don't make the solution any more complicated than what the problem calls for.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[Dougt115]

There are many questions that have been asked here for you to consider. Another possibility is that they are designing for continuous operation and one motor may be a redundant spare either active or passive as well as the loads could be in a redundant configuration too.

In any case I would recommend designing for the full 20 hp and until you are told otherwise no additional controls, clutches, torque matching, etc. These are simple units and if they are not in the design they may have a reason to not be included that they haven't been told.

 

[AltaIron]

Great insight guys.

Thanks again.

 

[mikekilroy]

I do not know the falk box; it may be a differential gearbox? I assume not from the comments so far.

But another idea not mentioned would be to just make it 20hp as Doug says for CYA, and run both IDENTICAL motors off the line or THE SAME 20HP VFD [put both motors on its output in parallel - with proper TOL], if variable speed required. Because of the asynchronous motor's slip, BOTH motors WILL share the load close enough to be no problem. They won't fight each other, the total energy used will be what the load required (assuming your description is right, 5hp each) and all pieces should last a very long time.

http://www.KilroyWasHere<dot>com

 

[geesamand]

Sounds like your customer either has a specific need and instead of describing the functional need, they decided to give you the first solution that came to their mind. Hopefully this is a proven design and you just need to get the details.

David