What type of steel are gears commonly made from?

[Andrew]

Originally Posted by Matt Adams

I've been trying to hunt all over the place to find out what sort of steel that boston gear, martin, or SDP makes their gears out of. I'm trying to perform some calculations on gear teeth strength, and I'm looking to find the yield strength for the steels that are commonly used.

I've assumed thus far that 40 KSI is probably a conservative estimate, but I'm pretty sure I've heard that the yield strength on gears could be even twice or three times that. I've also heard that you don't want to assume that sort of strength with gears in calculations, and want to use half or even a third of the true yield strength?

Any thoughts? Even just dropping a few materials would be fine- I'm aware of http://www.matweb.com

Thanks!

Matt

When I last needed gear material information from Boston, I just emailed them and they responded with the type of material and basic specs. I asked about their brass, so I cannot give you the numbers for their steel.

In my experience, yield strength is not a reliable number. It can depend on how the material has been worked. In fact, when the gear is made, it will be worked and therefore the yield point will vary.

Therefore, in calculations, you probably want to use the yield strength as a minimum guaranteed number. This is why you would pick a lower number than that quoted for the material. In other words, if yield strength varied +- 10%, you would want to choose the -10% value for failure calculations.

Other than that, the conditions that the gear tooth is going to experience are highly variable and the stress formulas include a number of approximations. You normally assume a safety factor in your calculations to account for both of these variations. Numbers from 1.5 to 3 are not uncommon.

As for variations in yield strength by material, some steels have 40 ksi. Without looking it up, I remember that 4140 steel is twice the "generic steel" yield. I seem to remember also that stainless steel tends to have lower yield strength than the non-stainless, high strength steels. That doesn't mean that there isn't a stainless, high strength steel out there.

You also need to make sure you mean "yield" and not "ultimate." At yield, the tooth will deform. At ultimate, the tooth will break.

Not knowing what you are trying to accomplish, I would recommend using 30 ksi for yield and a safety factor of 2. If your numbers say this isn't going to be good enough, I'd start to look deeper into the type of material and actual loading conditions.

[Matt Adams]

Originally Posted by Andrew

You also need to make sure you mean "yield" and not "ultimate." At yield, the tooth will deform. At ultimate, the tooth will break.

Not knowing what you are trying to accomplish, I would recommend using 30 ksi for yield and a safety factor of 2. If your numbers say this isn't going to be good enough, I'd start to look deeper into the type of material and actual loading conditions.

Just a couple of thoughts: In gears... I'd imagine that teeth deforming or bending aren't significantly better than breaking, since you'll obviously have pretty terrible binding issues with bent teeth.

The equations I'm using are the already conservative Lewis Bending equation, which assumes forces are acting at the tip, the length is the full depth, and the size is the width and height of the pitch diameter, treating it as a simple cantilever beam.

I'm trying to be conservative, since there will already be some dynamic forces involved as these gears will be engaging in the plane of rotation... but when you start compounding conservative values.. it gets to be a little crazy. I'm looking for something reasonably conservative. How you define that.. I don't know.

I appreciate your advice. Anyone else have thoughts?

Matt

[JessR]

Seems an unspoken part of your analysis approach is to account for the fact that the gear sizing will certainly be driven by the impact loading forces they will see, versus average loading over the service life.

The robot (& gears) have a service life of only few hours (maybe 4 to 20 hours).
You probably already looked at the Boston Gear specifiying material. They use the Service Factor approach & a cookbook, no calculation method. [ http://bostongear.com/ ] Not good for determining this sort of condition, hi impact with short running life.

A few things I would put in the mix for any calculations:
1. The best way to determine the impact forces is probably directly, hook up some instrumentation to a robot and crash it into a wall a few times.
If you want to do it from theory you need to pick some numbers & see how they work out.
Example - pick a number for a max. traction (lbs) your wheels might develop, and multiply it by.. 5? 10? for impact and use that to determing max allowable stress based on yield of the teeth surface (hertzian stress).
Get material specs from the supplier, Boston or whoever. For a first pass I'd use 30 ksi for steel or iron. When you get some actual values you can just scale them to the new yield.

[Joe Johnson]

Unless I know otherwise, I have used 60Kpsi as my yeild for the steel teeth I buy off the shelf.

It is not perfect, but it is probably close enough for my purposes.

As to using the Lewis equation, this is WAY WAY WAY concervative. I approximate the tooth as a beam with the load a the pitch line. I use Mc/I for my max stress condition.

I know these are pretty rough approximations, but they work for me (both in my day job and in 9 years with FIRST).

As to static/dynamic loads, I pretty much just design for stall torque with a whatever safety factor I feel I can sleep with.

Weight matters in FIRST. I try to balance a robust design with one that is not so overdesign it makes me loose a match because I could not put the right enough of sheilding around some soft bit of my bot.

Everything is a balancing act in Engineering.

Joe J.

[Ashley Weed]

Hey, its Greg P, yes, i am on ashley's delphi name, but AH! i do have an answer!

there are typically 3 common materials...

Brass
Steel
Iron

there are also some exotic materials used too...

lexan (plastics)
titanium alloys

yes, yes made gears outa lexan...go ask team 42 about their HUGE lexan gears...

technically, you could make a gear out of anything. the results might not be pretty, although i give you props for tryin'




~Greg!~