Screw pattern for max torque

RatLabGuy

RatLabGuy

You look like a monkey and smell like one too
Joined
May 18, 2005
Location
Churchville, MD
A question for the engineers here.

I'm working on a project (will probably post pics later) that involves rotating an assembly using a couple of servos. Servo has the typical HDPE wheel on top of the primary gear w/ screw holes.
I'm a little concerned about the stress/torque load on this attachment point, particularly b/c it is fairly small and the holes going through it are fairly small, and I cannot use particularly large diameter screws.

Here is my question:
Which design does a better job of distributing the load across the screw, given that the force is in a rotational direction (around the circle):
- screws in a "plus" style, w/ 2 on each leg, 8 total, or
- 8 screws put in a circle, like an 8-lug wheel hub.

Wobble should not be a problem b/c the assembly is supported by a ring of roller bearings. This is more an issue of the shear between the motor and resistance.

I'm leaning towards #2.

Really this is probably overkill and an academic discussion but hey, it's good for the noggin to think about these things
 
8 evenly spaced screws will give more consistent support around the object and less room for deflection. The 4 pairs of screws will leave you with greater unsupported areas. The only way I could see pairing them up being better is if it allowed the base material to have 4 strong legs instead of 8 weaker legs, and even then thats only better if the thing you are bolting to it adds no structural value.
 
This can go both directions but obvious either way depending on the shear and torque values.
You can spin the wheel of a jacked up car easier from the tire than by the lug with shear and torque being the major factor.
But on the other hand, if shear and torque isn't a concern, then the fact is that you can send all the kids back home crying with strawberries from the merry-go-round if you hop in the middle and turn it.
Been there and I remember I cried... But I was the LAST one holding on with feet straight out in the air by golly!!!:rockon:
 
Neither way is particularly great, because HDPE is soft and you'll have a hard time keeping consistent screw preload across all of the screws, and your stress distribution in shear isn't going to be ideal either. Depending on the surface area between the wheel and the gear, the shear load may not matter if the friction between the wheel and gear is high enough (then the clamp load is more important). It's just like a car wheel, where the lug studs aren't generally in shear unless the studs get loose and the clamp load between the wheel and the hub is lost.

The circle of 8 is usually the way to go though, as load distribution will be more consistent across all the screws versus having them on different pitch circles like you would with the cross pattern.
 
ai.imgur.com_tR6aspJ.gif



My bad...totally thought this was post was sex related (thought I might learn a trick or 3)
 
Fabrik8 said:
Neither way is particularly great, because HDPE is soft and you'll have a hard time keeping consistent screw preload across all of the screws, and your stress distribution in shear isn't going to be ideal either. Depending on the surface area between the wheel and the gear, the shear load may not matter if the friction between the wheel and gear is high enough (then the clamp load is more important). It's just like a car wheel, where the lug studs aren't generally in shear unless the studs get loose and the clamp load between the wheel and the hub is lost.

The circle of 8 is usually the way to go though, as load distribution will be more consistent across all the screws versus having them on different pitch circles like you would with the cross pattern.
Click to expand...
Since I feel like getting in a good engineering contest while drinking...

If neither are "particularly great", what do you suggest?
 
Falko said:
If neither are "particularly great", what do you suggest?
Click to expand...

I don't have any kind of valid answers without seeing pictures. "Not particularly great" just has to do with expecting to get a lot of performance out of fasteners that are clamping something relatively soft like HDPE under less-than-adequate conditions. If screw size and pitch circle aren't really adequate, my suggestion is to use something other than HDPE to fasten to the gear. This is all complete conjecture though because we haven't seen what we're actually talking about here, so it could be completely fine with no need for any improvement.

If my mental picture is correct, and if there actually is a reason to change something, I'm thinking a solution might be fastening a larger diameter metal plate to the gear face and then attaching the HDPE plate to that. That way the metal plate can get proper clamp load (and friction) to the gear, if that's what you're limited to using, and the the metal plate can transfer torque to the softer plastic turntable at a larger pitch diameter to lower any bending or shear force on the screws. Pretty much the same reason a lot of plastic gears and pulleys have a metal center hub to attach to. Remember that the farther you place a screw from the center of rotation, the lower the shear force is for a given amount of torque.

How much motor or gearhead torque are we talking here, is this thing transmitting a lot of power?

Falko said:
Since I feel like getting in a good engineering contest while drinking...
Click to expand...

Not a contest, just a brainstorming session. And you can't type and drink at the same time, so.... ;)
 
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8 in a circle as far from the center axis as possible.
 
I thought the friction of mating surfaces keeps them from rotating and then shearing the bolts.

In other words the bolts are only held in tension and the friction of the clamping force holds the two and transmits the turning force?

No expert here just my initial
Thoughts.

Only broken wheels studs I've ever seen is when they were not tight enough. Tight enough to center the wheel but not enough to transmit the torque through friction.
 
Mac5005 said:
Only broken wheels studs I've ever seen is when they were not tight enough. Tight enough to center the wheel but not enough to transmit the torque through friction.
Click to expand...
That makes a ton of sense.

I'm with this cat ^^^
 
OK so I don't know if it's HDPE, could just be nylon, it's the stock gearset that comes with this-
http://www.amazon.com/gp/product/B002HPUKS8?psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s01

I don't really know the torque load b/c I haven't finished building what it's driving yet. My back-of-the envelope guesswork is that the rating on the servo is plenty. The actual drive gear is metal.
Yes I'd rather use a larger diameter metal wheel but I don't have one and digging around for that kind of thing to match the specs is a pain.
The wheel is screwed to a piece of wood, so that's what the screws are leveraging.
Again though this sits inside the ring of a big roller bearing so the wood can't wobble.

Good point though about attaching a larger wheel/plate to it and screwing through that to distributet he load outward a bit
 
Mac5005 said:
I thought the friction of mating surfaces keeps them from rotating and then shearing the bolts.

In other words the bolts are only held in tension and the friction of the clamping force holds the two and transmits the turning force?

No expert here just my initial
Thoughts.

Only broken wheels studs I've ever seen is when they were not tight enough. Tight enough to center the wheel but not enough to transmit the torque through friction.
Click to expand...

You're absolutely right for most cases, unless it is designed to use drive pins or something like that because the friction or clamp load doesnt work for whatever reason.
Wheel studs are an interesting thing. They don't usually fail in shear if they get loose, they fail in fatigue because they now have no preload and are then subject to alternating load cycles as the wheel rotates. Fully alternating loads can destroy a bolt or stud through fatigue in very little time.
 
RatLabGuy said:
OK so I don't know if it's HDPE, could just be nylon, it's the stock gearset that comes with this-
http://www.amazon.com/gp/product/B002HPUKS8?psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s01

I don't really know the torque load b/c I haven't finished building what it's driving yet. My back-of-the envelope guesswork is that the rating on the servo is plenty. The actual drive gear is metal.
Yes I'd rather use a larger diameter metal wheel but I don't have one and digging around for that kind of thing to match the specs is a pain.
The wheel is screwed to a piece of wood, so that's what the screws are leveraging.
Again though this sits inside the ring of a big roller bearing so the wood can't wobble.

Good point though about attaching a larger wheel/plate to it and screwing through that to distributet he load outward a bit
Click to expand...

Ha-ha, I thought we were taking about industrial drive servos here, not little tiny RC servos. This changes things.
 
HITEC RCD USA

Torque ratings are on that page. With a servo of this type, I'd be really surprised if you can destroy that wheel before destroying the gear train. If you can transmit full torque with a rod linkage attached to a single hole, your application should be easy. I'd be more worried about wood attachment with screws that size. You might be able to get a coupler or something to replace the nylon disc.
 
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Really could go either way, if the torque load is low enough then the first option should be sufficient. On the other hand option two would be more ideal as you are sharing the load better than the first option. It may be best just to do a simple stress test in Solid Works too see if you really need the extra 4 screws vs. having only 4.
 
RatLabGuy said:
OK so I don't know if it's HDPE, could just be nylon, it's the stock gearset that comes with this-
http://www.amazon.com/gp/product/B002HPUKS8?psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s01

I don't really know the torque load b/c I haven't finished building what it's driving yet. My back-of-the envelope guesswork is that the rating on the servo is plenty. The actual drive gear is metal.
Yes I'd rather use a larger diameter metal wheel but I don't have one and digging around for that kind of thing to match the specs is a pain.
The wheel is screwed to a piece of wood, so that's what the screws are leveraging.
Again though this sits inside the ring of a big roller bearing so the wood can't wobble.

Good point though about attaching a larger wheel/plate to it and screwing through that to distributet he load outward a bit
Click to expand...


Ok now I'm curious... Wth are you building?
 
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