Mechanical Reliability

[Rick TYler]

Originally Posted by Andrew Blair

I've searched for awhile for a thread discussing simply mechanical reliability, and I can't really seem to find one.

I don't know about other threads, but our '05 bot is a pretty robust design.

1. All mechanical parts were designed with big safety factors. For example, we thought our motor would lift an arm with a tetra at a speed of N feet per second. We bought a big sprocket that would only move the arm at N/3 feet per second. This marginally slowed down our lift speed, but the arm didn't break and the motor didn't overheat or burn out.

2. KISS. Keep it simple. If you don't have a big budget and the help of a high-end corporate R&D lab, don't get cute. A simple bot that doesn't break will likely be more successful than a really clever one that you don't have the materials or tools to build strong.

3. Like the others wrote, do the math. We ended up changing some provisional specs after doing the physics analysis (it's handy having your mentor be the AP Physics teacher).

I'm sure others will have more clever suggestions for you soon. Good luck.

[billbo911]

Originally Posted by Rick TYler

2. KISS. Keep it simple. If you don't have a big budget and the help of a high-end corporate R&D lab, don't get cute. A simple bot that doesn't break will likely be more successful than a really clever one that you don't have the materials or tools to build strong.

3. Like the others wrote, do the math.

If I were to make any suggestion, these two mentioned above would be the two I would give.


I have seen a "Safety factor" mentioned a couple times. A ball park number for safety is to design for double what you think is the max. you will ever need. For example, if your arm will, at a maximum, have to support 8 pounds at full extension of 3 ft. Make sure you can support sixteen, or more. Also, don't forget to account for acceleration/deceleration forces of that mass. (Do the math!!!)By following the KISS principle, you may save a little weight. Use the extra weight to build a stronger arm.

[Ben Piecuch]

I've found that most of our failures have come not from the design, but from the manufacturing and assembly of our components. Using the wrong types of fasteners, or not using loctite, has caused many assemblies to come loose and fail. Misaligned holes, shafts, chains, etc... will also lead to early failure.

It's tough to spend the first 4 or 5 weeks designing, prototyping, and testing to find what works. Then spend the last week rushing to reassemble all your work, without much forethought to how long these assemblies need to last.

One of our stupid moments of this year came to light just a couple weeks ago at our last post-season comp. We noticed that our drive gearbox mounts had come loose, causing a misalignment with our drive chain. During the 6-week build, we had assembled all the mechanical components, then came back and installed all the electrical gear, including mounting panels and brackets. Unfortunately, the main electronical panel was installed directly over these gearbox mounting screws, and were impossible to get at without removing the entire panel. (Good times...)

Anyways... as most people have already mentioned, over-design as needed with respect to material selection, x-sectional area, and gear ratios. But, don't forget about the little details, than can (and will) come back to haunt you.

BEN

[ChrisH]

Originally Posted by billbo911

If I were to make any suggestion, these two mentioned above would be the two I would give.


I have seen a "Safety factor" mentioned a couple times. A ball park number for safety is to design for double what you think is the max. you will ever need. For example, if your arm will, at a maximum, have to support 8 pounds at full extension of 3 ft. Make sure you can support sixteen, or more. Also, don't forget to account for acceleration/deceleration forces of that mass. (Do the math!!!)By following the KISS principle, you may save a little weight. Use the extra weight to build a stronger arm.

Actually you are describing a STATIC safety factor. This assumes no acceleration (like being slammed by a robot). For my analysis on arms or lifting gear I assume the static load, add a 1/2g side load in the weak axis to account for getting hit. That gives a reasonable static load. I then multiply that by 4 to get a DYNAMIC safety factor. Actually the impacts we see will rarely push it to twice the load, but it doesn't hurt too much to add a little more. We have never broken a structure this way.

ChrisH

[Alex.Norton]

actually the force in that part of the engine is still not that large. The moving parts of an egine weigh almost nothing because that engine will turn faster if it is lighter so the rotation of the engine alone isn't going to cause the largest force in the engine. I mean if they actually weighed a lot then the engine would need a fly wheel in the sense of keeping the engine turning (the starter would have to turn the engine in a different method).

The largest force in an engine will come from the expanding gas in the piston causing the engine to turn. In my fathers car the engine is a 4 piston engine meaning that the torque put out is constant and only one piston is firing at a time. the engine has 205 ft lbs. of tourqe so that is the amount each piston puts out while firering. The distance traveled by each piston is 3.042 inches. divide that by 2 to get the radius. So the piston is pushing on a 1.701 inch lever. then divide 12 by that and mutiply by 205 to get the number of pounds on the be 1446.208 pounds.

Also just think that if the force exerted by the pistons of the bearings was 10 tons then engine would take a very long time to reach red line and most engines only take seconds.

Alex

[KenWittlief]

Alex, what is the angular acceleration of the connecting rod bearing journal, when the engine is spinning at 5,000 RPM, assuming your crankshaft is what? 1.5" radius (that sounds pretty small, I think an old VW crankshaft had a 3" radius)

and what is it at redline RPM?

another way to calculate it, the piston accelerates towards the crankshaft, then slows and accelerates the other way, at 5000 rpm, what is the force just moving the piston back and forth?

Edit: I did some quick calculations.

for an engine with a 4" piston throw (a 4" crankshaft diameter at the center of the bearing journals)

at 5,000 RPM, the piston and connecting rod are accelerating back and forth with A = 37,036 ft/S^2 (1157 g's)

and the angular (centripetal) acceleration of the connecting rod where its attached to the crankshaft is A= 45,654 ft/S^2 (1426 g's)

If your piston and connecting rod together weigh 1 pound, the force for the linear motion is about 1,100 lbs. The centripetal force depends on the weight of the connecting rod held by the bearing. If its a half pound then the force is 710 lbs - so we are close to one ton, and this is only with the engine spinning freely (not supplying any torque to the wheels).

the forces increase by the square of the velocity, so at 10,000 RPM (a high performace engine) you are looking at 4 tons of force on that one bearing, again, just to spin the engine parts themselves.