Our climber has gone through a late design change, and instead of being driven by chain and sprocket, is now designed as direct drive through a 1/2" hex coupler. We haven’t built it yet, but I’m concerned that this is going to fail. Unfortunately, I don’t know how to calculate the load rating on a coupler like this (calculating the load on a chain is much easier), so I thought I would see if anyone here had any thoughts on whether this would work.
Specs on climber:
Drum winch with 1.875" diameter with 1/2" hex drive. Coupled through Vex 1/2" hex coupler to 100:1 Versaplanetary (4:5:5) powered by 2 775pro’s.
Assuming it’s the Vex hex coupler, it’s a pretty beefy design. I don’t think it’s necessarily going to be the failure point, but I haven’t done any math to support that, so that opinion isn’t worth a lot. I think it’s more than possible that you’ll shear the hex shaft though if it is an aluminum shaft. It depends on how it is loaded and supported, but that is certainly a place where you could see a torsional failure.
You also have a slight risk of failing a VP stage. You are just so close to the max load rating there, using two motors (making the assumption that it’s about half the max allowable ratio for 1 motor), that I would be concerned about that as well.
Would it be possible for you to do a (35) chain reduction between your VP and your climber shaft, and then change your climber shaft to 4140 steel? This would reduce the load on the gearbox significantly, and it would allow you to avoid using a hex coupler as the support for your robot’s weight.
Sorry, I guess I should have made that more clear. We would be using the Vex coupler that euhlmann linked to.
We were planning to use 1018 steel for the climber shaft. Would 4140 have a noticeable higher shear strength? Our original design used less reduction on the Versaplanetary and had a final reduction with 35 chain and sprockets, but it didn’t integrate very well, so the direct drive design was proposed. I know this current design isn’t nearly as robust as the previous one, at this point I just trying to figure out how bad it is.
Test it. You know how much your robot weighs, you know the radius of your winch drum, so you know the torque needed to lift the robot. Clamp a hex shaft in a vise, add a coupler and another piece of hex shaft, and apply torque with a torque wrench set with the safety factor of your choosing.
I just want to quickly point out that it’s unlikely you’re going to stall your climber, and current control can be added in an SRX. The VP load ratings are for stalled motors.
I think the most failure-prone parts of a climber this year are bending in the shaft, not necessarily torsion. The amount of torque on a 1/2" hex shaft, unless you are using an extremely large pulley, is small compared to the ultimate rating according to the VP rating guide (~120 ft-lbs). That means that if you’re using a 2" diameter pulley/drum, you’re only putting a load of about 1/5th the ultimate load rating of a shaft. Even including shock and acceleration loads, that’s not too bad.
For people who aren’t familiar with how those different properties relate to different loads, could you help fill that in?
I see that the Brinell rating for 7075 is higher, but does that mostly have to do with how the metal resists impacts? And does Unit Resilience and Yield Strength indicate how it well a rod would resist snapping under load? For instance, I see that 1018 has more Ultimate Resilience than 7075, but I know you are saying that 7075 would be a better metal in a winch application.
7075 nearly matches or exceeds mild steel in many of it’s structural properties, it is primarily just less wear resistant due to the softness of aluminum.
Aluminum will bend more than steel, though - modulus of elasticity of 1018 is nearly three times that of 7075 aluminum. We’re more worried about the bending of our winch shaft than torque limitations, so we’re going with a cold-rolled steel hex shaft. Ours produces a lot less torque than OPs (the axle **is ** the drum), so the situations are not quite the same.
“Brinell” is a scale for measuring hardness (another common scale is “Rockwell”), which describes how strongly the material resists deformation. A higher Brinell means a harder material, and there are some correlations between hardness and strength for some materials but it’s not universal across all materials. Not particularly useful for making materials selection choices in FRC, IMO.
Yield Strength describes how much stress the material can take before it begins to permanantly deform from its original shape. “Yielding”=“deforming”. A sheet metal brake creates bends by creating enough stress in the part to exceed its yield strength.
Tensile Strength (sometimes called Ultimate Tensile Strength, or UTS) describes how much stress the material can take before failure in tension (as opposed to compression). In this context, “failure” means “breaking”. For aluminum and steel, this will always be a higher value than the yield strength.
A simple example of the difference between yield and tensile strengths: if you take a shaft in your hand and bend it, and it keeps that bent shape when you let go of it and it didn’t break, you’ve exceeded its yield strength but not its tensile strength. If it breaks, you’ve exceeded its tensile strength.
Modulus of Elasticity (sometimes called “Young’s Modulus” or “Elastic Modulus”) describes the material’s resistance to non-permanant deformation in tension or compression (rotation is a different constant). This isn’t “stiffness”. Stiffness is a property of a component (taking into account shape and material), elastic modulus is a property of a material. You’d plug a material’s elastic modulus into a bending equation that will also take into account the shape of the part.
“Impact resistance” is a measure of toughness, or the material’s ability to absorb energy without fracturing (breaking). It takes into account strength and ductility. Watch out for how this is reported - if you’re using MakeItFrom, you’ll get it from the Unit Resilience and Ultimate Resilience values they provide. If you check out their glossary(which you should!), they define Unit Resilience as the material’s ability to absorb energy to the point of yielding (deformation), and Ultimate Resilience is how they describe the material’s ability to absorb energy to the point of failure (breaking) (which is really the traditional definition of toughness).
Is that helpful? Thre’s obviously a lot more to materials selection than just properties (shape, loading, temperature, cost, weight, etc) but I think this should be enough to answer your questions.