Increasing input HP to gearbox.

[jrexrode]

Hi all. I'm hoping someone here can give me some guidance as I don't have a lot of experiencein working with gearboxes.

I have a 750:1 gearbox being driven by a 1HP motor connected to a VSD. The input HP as defined by the data tag is 1.38 HP. The motor runs at 10 Hz raising the load until it reaches its destination, but the drives faults out on overcurrent before the load reaches its destination. Can I safely replace the 1 HP motor and drive with a 2 HP motor and drive without damaging the gearbox? If this isn't an option, would changing the sprocket sizes to allow the motor to run a more optimum speed help?

 

[drawoh]

jrexrode,

You should not exceed the power rating of your gearbox. At the very least, chat with the gearbox manufacturer and see what they think.

If you can change sprockets to increase your total gear ratio, you reduce your power. That ought to solve your problem. Your speed will be reduced accordingly.

You should take a close look at the overall design, and chat with the original designer if necessary. I am not aware of any safety issues with overloaded gearboxes. Your gearbox may have been correctly sized for half your load. If anything else was correctly sized for half the load, you have a safety problem.

JHG

 

[jrexrode]

Thanks drawoh for the reply. I have spoken witht he gear box manufacturer and they are ok with increasing the motor power to 1.5 HP but not any higher. I've just tried increasing the motor RPM to get it out of the low frequency range, thinking that running that low of an rpm was contributing to the problem. No luck on that as it didn't make a significant difference in the current draw of the motor.

 

[dvd]

If your horsepower calculation predict that you should have enough power to raise the load, you might want to look at the drive configuration, in particular how far the motor is from the vfd. Maybe post a discussion of your problem on the electric motor forum.

 

[eromlignod]

The load on your gearbox is all about the *output*. Motors are rated at the maximum *possible* power they can supply.

You could put a 300hp motor on the input and as long as your output only requires 1hp, that's all the motor, or the gearbox, will ever see.

If you're faulting out your motor, then your load is probably greater than its 1.0hp rating. If you go to a 2hp motor the gearbox may or may not see all 2.0 hp. It's possible that the load is somewhere between 1hp and the 1.38hp rating of the gearbox, but if it's closer to 2hp, then you're going to overload your gear train. Even if it falls in the gearbox's range, your probably pushing the envelope.

You need to determine what your ouput power requirements are, then divide by the efficiency of the gearbox and include a safety factor.

Don
Kansas City

 

[drawoh]

jrexrode,

If you increase the motor speed at a given torque, you are increasing the power and making your problem worse. Power is force[×]velocity and torque[×]rotation speed.

You did not say what kind of motor you have. If it is a DC motor, your maximum torque is at stall, your maximum power is at half of no-load speed, and your maximum efficiency is somewhere in the vicinity of 2/3 no-load speed. Your torque is approximately a function of current, and your voltage is approximately a function of motor speed.

If it is not DC, the following remarks are less correct, but still pretty valid.

If you were to double your gear ratio and run the motor twice as fast, you would be producing the same power at very approximately half the torque. The gearbox efficiency would affect this. Your current goes way down. This might solve your problem without affecting speed.

You really should talk to the original designer. You should not modify bits and pieces of a system if you do not completely understand how it works.

JHG

 

[jrexrode]

Thanks for all the responses. I've just about came to the conclusion that the best solution may be a different gearbox and motor combo. Just for y'all to ponder some more if you want to, the application this is being used in is a steel accumulator holding about 250 feet of material for a rollforming line. The unloaded (accumulator unthreaded) amperage draw of the motor is about 1.2 amps when its raising or lowering. The loaded amperage draw is 2.1 to 2.2 amps when its accumulating material (raising). Have another unit exactly identical as far as drive equipment but instead of the material being steel, its aluminium. So far, we haven't had any problems with overcurrent. I'm thinking that the added weight of the steel material is the culprit.

I was hoping for a simple, less costly solution to this problem, but that hope is fading fast. Again, thanks for the replies.