Gear Box Lock-up 1

[A514]

I am running a system that incorporates a 1.50 H.P. A.C. motor running at 1160 RPM's This is directly coupled into an electric clutch that runs upto 24 v.dc. and that generates up to 36 ft-lbs of torque at 24 volts and can be adjusted downward. From the output of the clutch is a 12 tooth sprocket running a drive chain into a 21 tooth sprocket. Where the 21 tooth sprocket is mounted is a line shaft that runs into 2) Cone-Drive model 30 gear reducers that are 60:1 ratio's. The output shaft changes direction and a pinion gear is mounted vertically. The pinion gear is a 4.00" P.D. that drives one set of gear racks and pulls a set of arms. Each set of arms weights about 800 pounds that slides into the frame.The RPM's of this shaft is around 11RPM's. I ran my calculations and I figured I only needed 3/8 horsepower motor based on a 0.3 coefficient of friction. The arms travel inward or outward until they hit a hard stop at that point the electric clutch slips. The clutch starts out at high voltage and drops down to about 12 volts when running. The problem is when after the arms hit the stop the motor stalls and I can't cause the arms to change directions.It's like I need more torque to break the arms free. It's almost like the gear box locks up and I need even more torque to drive the arms in or out. I was wondering how much break away torque I need to reverse directions with theis 60:1 ratio gear box? Also would lowering the ratio reduce the breakaway torque into the gearbox? Thanks in advance.

 

[MikeHalloran]

What you're basically doing is driving the racks against a stop, then using 1.5HP and the greatly magnified inertia of the motor to drive a self-locking wedge behind them.

You can't drive the wedge back out with the same power.

Instead of adding more power in the reverse direction, just don't drive the mechanism against the stops at full power and full speed. Add switches near the stops that will cut off the power and let the motor coast, or dynamically rearrange the connections so it runs at reduced power.

[ By the latter I mean this: If you happen to be using a dual voltage motor at the lower voltage, two field coils are connected in parallel. Open one for reduced power. ]

You might be able to make it work as is by disengaging the clutch and running the motor up to speed in reverse before engaging the clutch to break the mechanism free. More likely, you'll break the clutch.




Mike Halloran
Pembroke Pines, FL, USA

 

[A514]

Thanks Mike,
I have a 440 Volt 3 phase motor running at full voltage. I have an electric clutch that is adjustable. When running it runs at 12 volts at that point is when the arms hit the stops. When reversing direction the clutch uses full voltage 24 volts momentarily, I was hoping that would be enough to break it free. The clutch grabs but the motor stalls. I guess I can also lower the voltage of the clutch more if it will move the arms. These arms of this unit will strike an object because it is used as lifting arms. The main problem I have is if the operator is close to the object and closes on the load, the clutch can hit it at full voltage and then lack the power to reverse it. Perhaps I can reduce the speed of the motor to start, or use a centrifical type clutch. I've built similar systems in the past, however I used 240 volt Direct current series motor and can break the the load free. Excellent starting torque with series wound motors. If you know of any type of controller I'm all ears. In the mean time we are trying to reduce the friction of the arms so we don't need as much power.
Thanks again,
John

 

[MikeHalloran]

I guess you'll build the next one with a DC motor.

Maybe this one, too; I'm fresh out of magic bullets.

...

Well, maybe if you put fingers on the arms to sense proximity to the load, and had those run the clutch...



Mike Halloran
Pembroke Pines, FL, USA

 

[A514]

Thanks Mike,
I've noticed a few problems, one with output shaft alignment the other with the gear box efficency. Right now i'm using a 60:1 gearbox with a 4% back drive efficency. I ordered a 30:1 gear set and should have them installed in about a week. Your previous post gave me an idea about the wedge effect. So I looked to the gear box. By changing ratios I went from 4% to 29% back drive efficency, I slowed the unit up on the input side. So I'll keep you posted.
Thanks,
John

 

[A514]

The gear box worked, the ratio of 30:1 worked. When you run the drive into a hard stop you can run the arms in reverse. So thanks for your help Mike.