Application of the Month
When Worm gearboxes need to be backdriven, disengaged or torque limited
May, 2013 - View all Application Examples
Worm gearboxes and speed reducers can be very cost effective torque multipliers, especially as ratios climb above 10:1 or 12:1. They can typically achieve ratios up to 100:1 in a single gear stage which drastically reduces the component count of other gear types with these ratios. That, along with a worm shaft just being cheaper to produce, makes for a compelling low cost solution.
Worm and ring gears have other advantages as well. Because of their sliding action, rather than rolling, they can be among the quietest gearboxes. They are often used in medical equipment where there is patient interaction. There’s clearly enough mental stress during medical tests without hearing the machine rattle and clank.
Worm speed reducers also have relatively good shock capacity, actually much better than most other gear types. There is more potential “tooth” contact surface to absorb shocks. During crashes or e-stops the direction of force on the worm shaft is slightly deflected, reducing the direct axial forces seen by the input bearings.
What can be another huge design advantage of the worm gearbox is that, depending on size and ratio, it can’t be backdriven, or at least not easily. This is a great benefit in a variety of applications where relative position needs to be held or vertical loads can’t be lost.
It’s because of one of the negatives of the worm that the previous advantage even exists. That pitfall is the relative inefficiency of worm gear reducers compared to almost any other gear technology. Worms are just plain inefficient. They don’t coast well and in reverse they really don’t want to slide at all.
So what happens when a customer has a low cost machine and needs to move the load when power is lost or the drive system is locked up? The worm is a great solution for simple conveyors, doors, etc or actually anything that needs mechanical advantage but not a lot of precision. But when power goes out things have to get disassemble before they can get moved. That can be a pain!
And that is the exact situation that a client recently brought forth. His product was motorized gates and doors for premium residential and corporate security requirements. But it could have been any application where a load is driven but its critical that load can be moved manually in the event of a power loss or if the motor froze.
The customer knew enough that he couldn’t push worm driven gates back against the box. So he asked whether there was a way to disengage the driven side so the load could be manually moved.
Our simple answer was “Yes there is”. We could accomplish the disengagement of the output shaft with our Varvel series TL (PDF - 164KB) torque limiting and safeguard device option.
While it is true that the series TL option was primarily intended to provide an internal torque limiter for the protection of the load and the gearbox, its design allows it to be dialed down to almost zero residual torque basically disengaging the output shaft from the rest of the gearbox.
There are two distinct design types however both feature several compression washers that drive the output shaft through friction. The concept is that when the required torque exceeds a predetermined level the compression washers then slip. This protects the load against hard shocks from jams due to overdriving the box with motor torque or inertia from the system.
The torque level where slippage occurs can be manually adjusted up or down relatively easily, perhaps with a small hand wheel. And as previously stated the torque setting can be adjusted down to a level that virtually disengages the load. Therefore, in the event a manually movement is necessary the operator could actuate the torque limiting device and easily move the load, in this case forcing the security gate open.
One issue for our security gate customer that wouldn’t be relevant for other applications needing the ability for the output to be driven is the gearbox size. Very intermittent duty applications can often downgrade the size of the gearbox because of the limited running life expectancy. A 0.8 or even 0.5 safety factor (S.F.) can often be used for these applications because there is plenty of gear strength and the bearings will never wear out with such limited usage.
Unfortunately, torque limiter designs are typically matched to the gearbox running torque so as not to exceed its catalog rating. Since torque limiters are dumb, i.e. they slip at a set level regardless how often or how fast the box is used, the gearbox selected would have to be based on the torque limiting range and not the gearbox capacity. In other words you can’t increase the “effective” rating of the gearbox because of low duty cycle.
Another issue would be drive security. Worm gearboxes are often used because of their protection against back driving. Gates can’t be opened and inclined conveyors won’t flow backwards from heavy loads. So some consideration for guarding the access of the torque adjuster would likely be important.
There are many applications where the capability of this option can protect against overload or be used to disengage loads. Be creative when designing your drive system. Ask questions of your supplier. Not every option available can be shown in a catalog or presented in an application guide. If you find a supplier who can understand your unique needs and has a shelf full of solutions, along with the know how to apply them, then you’ve found a winner.
Mmmm, that sounds like DieQua Corporation.
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