Ability to backdrive a gear reducer

[SevenToFive]

How can I determine if a single worm gear reducer will backdrive? The gearbox has a 15:1 ratio which has around 90 percent efficiency. The gearboxes output shaft also turns a pair of sprockets that creates a 3:1 ratio. This in turns 8" inch diameter steel wheel on a 28000lb trolley, that rides on steel rails on a level surface.
The user wants to verify that if the trolley is not properly centered over it's load that it can center itself.

Any help is greatly appreciate. Thanks to all of those who reply.

 

[Dr.D]

In general, worm drives will not backdrive. The friction prevents this.

What the backdriving of a worm has to do with centering a trolley is not fully evident.

 

[CWatters]

I'm not a mechanical engineer but I played with Mechano a lot as a kid in the 1960s' if that counts! I never manage to back drive one of their worm gears but it can be done if the lead angle is high enough.

Google found this page
http://www.roymech.co.uk/Useful_Tables/Drive/Worm_Gears.html#SelfLock
which says..

Referring to the above graph , When the gear wheel is driving the curve points intersecting the zero efficiency line identify when the worm drive is self locking i.e the gear wheel cannot drive to worm. It is the moment when gearing cannot be moved using even the highest possible torque acting on the worm gear. The self-locking limit occurs when the worm lead angle ( γ ) equals atan (μ). (2^o to 8^o )

 

[CWatters]

The user wants to verify that if the trolley is not properly centered over it's load that it can center itself.

If that needs the worm to be back driven then +1 to what Dr D said. It's very unlikely unless the worm is designed for that. Usually people are concerned about accidentally back driving a worm which can happen unexpectedly especially when there is a lot of vibration.

 

[Dr.D]

I cannot imagine a high angle worm used in a power transmission application in the real world. In a toy, perhaps, but not likely in a trolley.

 

[CWatters]

I agree.

 

[SevenToFive]

Dr.D and CWatters, thanks for the input. The gearing company that makes the gearbox has told me that the lower ratio units such as this one is capable of being backdriven, the only way to stop backdriving is by installing a brake. However in this case our customer would want to be able to backdrive, so if the operator goes slightly beyond the load that the trolley lifting the load will be able to center itself.

If the 28000lb trolley is riding on steel rails it should be able to love rather easily since the steel on steel coefficient of friction is only 0.001 to 0.002. With a 8" wheel one should only need 224in-lbs.

Perhaps the real question that I should be trying to figure out is how much torque is needed to turn the gearbox from the output shaft and turn the gearbox and 3HP motor?

 

[CWatters]

Did the manufacturer mean they could be deliberately back driven or that it could happen accidentally? Big difference.

 

[CWatters]

I still have no idea what you mean when you say "centre itself"? Got a photo?

 

[CWatters]

Do you have to use a worm gear?

 

[Nidum]

I suggest that this discussion does not proceed further until we have a clearer idea of what is going on here .

This is heavy equipment with potential dangers and the seeming need for the powered gear box to back drive sometimes and not others depending on the load alignment and the whim of the customer is alarming .

@SevenToFive - Please post a clear diagram of what the configuration of this equipment is so that we can understand this problem properly .

 

[SevenToFive]

Well hopefully this will help. We are rebuilding this trolley as they call it that rides on a level structure that has a number of doors or gates as they call them. Currently they manually push the trolley over to the desired door, connect a cable to the gearbox in the center and raise the door. Now they want to install a 3HP motor and gearbox to move this trolley across the top of the structure since they already have power for the motor on the center gearbox. The gearbox sits on top of the trolley with the output shaft covered with a sheetmetal shroud and chain/sprocket combination is inside the structure of the trolley, so it is not actually exposed. The center gearbox is not being changed as they claim it works perfectly.
Now the gearbox manufacturer states that gearbox can be driven in either direction as long as it comes to a complete stop before changing directions. They also do not guarantee that their gearboxes will not not backdrive, as they say the condition always exists. However in this case we would want to be able to backdrive so the operator can have the center gearbox inline with the center of the door that they have to open and close. My drawing is probably over exaggerated.

Does this help explain what is trying to be achieved?

 

[CWatters]

Can I just check... So the plan is to use the motor to get the truck in roughly the right position then manually move it the last bit by hand to get it exactly over the required gate?

 

[256bits]

the gearbox manufacturer states that gearbox can be driven in either direction as long as it comes to a complete stop before changing directions

we would want to be able to backdrive

they want to install a 3HP motor and gearbox to move this trolley across the top of the structure

Perhaps I am missing something here, but what is the matter with having the cart move backwards and forwards under power. Unless your track is circular, you will need a reversible motor, or some other way to get the cart to move from door to door ahead of it, or behind it.
And a brake to hold it in position when centered, if movement of the trolley when stopped is problematic.
Does the cart move so fast that the operators will not be able to click-click on the control switch to get the trolley into position?

Working with wormgears, the term 'backdrive' has a particular meaning, and you and the manufacturer can converse and understand.
For the client not familiar with worm gear terminology, backdrive could mean anything. To them it may mean 'reverse.'
Could the 'problem' that you are attempting to solve be a matter of miscommunication?
A good solution can inherit from a well defined problem.

 

[CWatters]

I also wondered if someone was confusing "reverse" with "backdrive".

 

[Nidum]

The trolley can be placed in approximately the right location over the door using the powered positioning drive but there is a problem in that the centre line of lift of the door lifting mechanism can be a little out of line with the ideal centreline of lift of the door .

The available means of achieving final alignment within required limits are :

Use the powered main positioning drive again but with better control .

Manually push the trolley along .

Couple the lifting chain regardless and allow the lateral component of the lifting force to pull the trolley into place .

Is this correct ?

 

[SevenToFive]

That is correct Nidum.
I agree 256Bits, you would think they would have some better control on motor, like a potentiometer or something. And you're correct it will have to go forward and reverse, since the track it rides on is a straight line a couple hundred feet long.

 

[Nidum]

You can actually buy motor/driver units off the shelf with fixed or walk around control boxes that give the user complete motor/positioning control . They are not even very expensive by industrial standards .

Standard installation on equipment like works gantry cranes .

Question now is whether you try to educate the customer about a better way of solving their problem or try to make the best of what they are currently asking you to do .

If the latter then you are in difficulties .

Your worm and wheel gearbox won't backdrive in the circumstances that apply in this design so to move the trolley by hand or by pulling over with the lifting chain you are going to have to disconnect the drive somewhere . Could be done but likely to be an untidy and expensive solution .

Personally I would try educating the customer . A proper motor control system is going to be very much easier - and probably much cheaper - to implement .

 

[CWatters]

Perhaps some strategically placed optical sensors would be enough to stop it in the right place or even automatically centre it.