Gearbox Reverse Drive Question
Gearbox Reverse Drive Question
(OP)
Lets take a gearbox of ration 4:1. 4 in = 1 out.
But say we were to drive the gearbox in reverse, ie 1 in = 4 out. As a rule of thumb, taking an average friction ratio for gearboxes, what is the maximum gearbox ratio before the gearbox will bind up under the friction of the gears?
But say we were to drive the gearbox in reverse, ie 1 in = 4 out. As a rule of thumb, taking an average friction ratio for gearboxes, what is the maximum gearbox ratio before the gearbox will bind up under the friction of the gears?





RE: Gearbox Reverse Drive Question
RE: Gearbox Reverse Drive Question
RE: Gearbox Reverse Drive Question
Barry1961
RE: Gearbox Reverse Drive Question
RE: Gearbox Reverse Drive Question
One can indeed back drive a pinion with a rack. However, as the pinion becomes smaller, the force required to generate a given torque on the pinion becomes larger, and eventually reaches a level sufficient to fracture a pinion tooth.
For a single gear pair, there is a ratio beyond which the teeth are not strong enough to transmit the loads on the small gear.
For planetary gearsets, the problem becomes more difficult because the gears must be precisely matched to make them share the load.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: Gearbox Reverse Drive Question
For years, I too thought that an involute gear mesh had pure rolling friction until I happened to buy Earl Buckingham's book, "Analytical Mechanics of Gears." To quote, "Thus when two involutes are acting against each other, a combined rolling and sliding action takes place between them because of the varying lenghts of equal angular increments on the profiles." He then goes on to analyse the sliding part in detail. It is small (usually less than 1%) but not insignificant and is the primary efficiency loss mechanism in gears.
In fact, the sliding is smaller by half between a rack and a pinion than it is between two "pinions" of equal size.
RE: Gearbox Reverse Drive Question
in the arc of approach and the other
in the arc of recess. The first
does cause friction. Luckily it is
often a small part of the mesh.
RE: Gearbox Reverse Drive Question
I now have more information. The gearboxes are 255:1 planetary 3-stage. Guys, putting individual gear teeth meshing friction, arc of approach, etc aside for a moment, is it possible to back drive a 255:1, 3-stage planetary gearbox?
RE: Gearbox Reverse Drive Question
see why it would not be possible.
RE: Gearbox Reverse Drive Question
<"Metalguy. Ball park figure. Lets say single stage planetery. Real rough idea.">
I don't think you can get much over a 10:1 reduction with a single stage planetary gear set. A 10:1 is very easy to back drive.
The largest ratio planetary gearbox I have back driven was a two stage 100:1.
On a side note, if you are driving the gearbox with a DC or servo motor you will probably have to disable the dynamic braking to back drive it. Most DC and servo drives default to dynamic braking mode when power is off.
Barry1961
RE: Gearbox Reverse Drive Question
RE: Gearbox Reverse Drive Question
RE: Gearbox Reverse Drive Question
For example it is common to get a two stage 20:1 ratio with a 4:1 and a 5:1, not two 4.47:1.
I don't see why having three sets of 6.34:1 would not be possible, just not likely.
You might be able to get the mfg to back drive one on the bench and measure the torque for you. It would probably be close with any three stage box. You could also measure the torque to back drive your motor then multiply it by the ratio. With such a high reduction I would try to measure the static/break away torque also.
Barry1961
RE: Gearbox Reverse Drive Question
Or could you expect something like i.e. 10:1, 12.75 & 2:1 for planetary gearboxes?
RE: Gearbox Reverse Drive Question
If all they were worried about was torque rating I would guess they would want to stay in the middle, 5:1, as much as possible. But of course noise is a big issue these days so they probably size the first stage to a quiet ratio even if they have to sacrifice a little torque and efficiency to make it quiet. I don’t know what the quietest ratio would be or if it would be the same across brands. My gut tells me the higher the better for noise but it is probably not that simple.
Some of your less precision industrial boxes such as Rex PlanetGear may not care about noise since they are not designed to operate much over 1750 rpm and are not used in quiet environments like hospitals. I would assume for this grade of box they would size the ratios based on torque and cost only. They may be willing to sacrifice a small amount of torque to use a gear set that they need more volume in to keep cost down or a gear set that uses less material or cheaper bearings or easier to manufacture.
There are good engineers who spend their entire career figuring out the best design for a planetary gearbox and still learn new stuff everyday.
Barry1961