Winch Motor Stalls or Wire Rope Breaks? 1

[djt1967]

Our customer has a problem unrelated to the work we’re doing for him but being kindly souls we thought we may be able to answer his question…unfortunately we couldn’t so I’m humbly asking if anyone here could shine a light on the problem…..

The customer has a winch that lifts a load weighing 25kg up a shaft 30m deep. A 2mm Dia wire rope goes up the shaft over a guide pulley and down to the winch reeling drum. There is a load cell on the guide pulley.

The question is this. “If the load cell fails and the load being lifted gets snagged on the steel work as it is lifted will the wire rope break before the motor gearbox on the winch stalls / trips out?”

Wire Rope 2mm Diameter with a Breaking Load of 230kg

Winch Drum 300mm Diameter.

We are still waiting for the information on the Motor / Gearbox but if someone could point us in the right direction as to what formula we could use to calculate this and also what specific information we would require for the Motor / Gearbox. Thanks in advance for the any help provided.

 

[sacem1]

With out a power data on the winch motor and the speed that the cargo is conveyed it is impossible to give you an answer.

As a general rule from a winch builder point of view you will always stall the motor first and the rope should not fail under any load that the winch can pull.

The reason is this:

The winch has its overload relays designed to stop the elevation at 25% overload from the nominal load capacity of the winch.

The motor with the aid of all gear reductions in the winch is usually designed to handle a 50% overload from nominal capacity of the winch in order to absorb starting torques with out overheating.

The cable has a maximun strenght that usually is at least 500% the nominal load capacity of the winch in order to absorve fatigue and decay without jeopardizing the persons involved with its work and sudden overloads due to jams or sudden detentions (as in your question).

So your winch should stall if the overload trip does not work and the load gets caught somewhere and produces a much larger strain than the 25 cargo load.

Regards

SACEM1

 

[desertfox]

Hi djt1967

More information is needed as stated by the other poster, however if you know the breaking load of the cable and the winding drum is 300mm diameter then the max torque that can be exerted by the motor before the cable breaks is:-

max torque = (winding drum /2*1000)* 230kg * 9.81 = 338.5Nm

So if the load snags the cable will break theoretically if the motor can deliver the above torque.

regards

desertfox

 

[sacem1]

The formula that desertfox gives will be correct for the motor ONLY if the motor is directly coupled to the winding drum, if it has any gearing involved you will have to include the reduction in speed (increase in torque) that the drum will be able to deliver as tension to the cable.

As a rough gide the increase in torque will be directly proportional to the decrease in turning speed of the winch drum compared to the speed of the propelling motor, an efficiency factor should be considered but for safety calculations you can use an e.f. of 1.

Regards

SACEM1

 

[desertfox]

Hi djt1967

Perhaps I should have stated differently in so much as the torque figure I calculated would be the MAXIMUM torque the actual winding drum could see before the cable failed if it got snagged.

regards

desertfox

 

[djt1967]

Thanks for the help everyone. I have some more information.
The Motor is a SEW Eurodrive S62R42DT71D8/2 BMH.HWS.B31A/270 which is 2/8 RPM Output and 0.06/0.25kW.

Then there is a David Brown A410BE405R 40:1 worm reduction unit.

I will update when I get more info.

Thanks again for the help it's most apreciated.

 

[desertfox]

Hi djt1967

Just confirm what the output speed is 2 to 8 rpm?

regards

desertfox

 

[djt1967]

Hi desertfox,

Yes the output speed is 2 to 8 rpm apparently.

Cheers

djt1967

 

[desertfox]

Hi djt1967

Can't post fully now but with that motor and gearing the motor can produce far more torque to the drum than the
cable needs to snag and break.
You need to find out what protection is on the motor, ie what the load sensor setting that will trip the motor?

regards

desertfox

 

[sacem1]

Seems that you have a 2 speed motor with 2 HP ratings:
0.08 HP @ 2 RPM AND 0.33 HP @ 8 RPM

with a 40:1 reducer and a 300 mm winch drum, assuming that the wir rope hangs as a single line we will have the load being raised at the following speeds:

300 mm x 3.1416 = 942.5 mm perimeter of the drum raised per 1 rpm

2 rpm divided by 40 (reducer) = 0.05 rpm

ergo we have 942.5 mm x 0.05 rpm = 47.125 mm per minute raising speed = 0.155 ft/minute

load is 25 kg = 55 lbs

the power requiered to raise the cargo is:
0.155 x 55 / 33,000 = 0.000258 HP asuming e.f.=1

(1 HP = 33,000 lbs ft/min)

doing the same calcs with the load the wire rope resists 230 Kg = 506 lbs

0.155 x 506 / 33,000 = 0.00237 HP with e.f. 1

Then if we asume a 0.8 eficiency as probable with this setup, the power for breaking the wire rope would be:
0.00237/0.8 = 0.00297 HP

Which is much less than the power you have availiable which is 0.08 Hp ALMOST 27 times what you need to break the cable.

ERGO: The cable DOES NOT RESIST THE PULL OF THE WINCH IF IT GETS STUCK.

Regards
SACEM1

 

[desertfox]

Hi djt1967

I was mistaken earlier the max torque for the motor is about
286Nm and 298Nm respectively which is lower than my earlier
calculation.
However the safety factor under those circumstances seems low and with cable deteriation over time it may become a problem.
It maybe worth while increasing the size of the cable.

regards

desertfox