Elevator Design - Rack and Pinion Cascaded - 772

[Ether]

Originally Posted by akoscielski3

when it was down its 31" from bottom, and when it's fully extended it's 57" from bottom of the Elevator.

How did you calculate that you could lift 28 pounds 26 inches from a dead stop in 0.22 seconds with two 550 motors ?

[Madison]

I would avoid using a rack and pinion in this manner because I am not absolutely confident that we could keep both motors synchronized well enough to avoid binding. One motor spinning at a slightly different speed than the other will eventually cause its side of the elevator to be one or more rack teeth off and the elevator may eventually bind.

[Jared Russell]

Originally Posted by Madison

I would avoid using a rack and pinion in this manner because I am not absolutely confident that we could keep both motors synchronized well enough to avoid binding. One motor spinning at a slightly different speed than the other will eventually cause its side of the elevator to be one or more rack teeth off and the elevator may eventually bind.

Moving the rack to the rear of the uprights and using a shaft to connect the two motors may be an option.

[billbo911]

Originally Posted by Madison

I would avoid using a rack and pinion in this manner because I am not absolutely confident that we could keep both motors synchronized well enough to avoid binding. One motor spinning at a slightly different speed than the other will eventually cause its side of the elevator to be one or more rack teeth off and the elevator may eventually bind.

Madison is absolutely correct on this point!

The workaround should be fairly simple.
Assuming the carriage will carry some sort of manipulator on the front side, move the drive mechanism to the rear. Tie both drives together with an axle. This will keep both motors/pinions synchronized. Just make sure you can adjust the vertical position of one rack so any misalignment of either pinion can be taken into account.

The question I immediately had when looking at your design was: "How do you plan on holding the lift in any position. Do you have a locking mechanism? Or, were you planning on stalling the motors? If stalling the motors is your plan, you really need this thing counter balanced properly.

[akoscielski3]

First off I never mentioned putting constant force springs on the elevator to eliminate almost all of the weight. and the 28lbs included the outside rail, which is not being lifted at all, the mass that is being moved would be almost 0, if not positive from the springs. I talked to Adam Heard last week about their Elevator and the springs so I learned a bit from that, but I may be wrong.

I attached a picture of my Calculations from JVN's Calculator, am I possibly doing it wrong?

[akoscielski3]

I didn't see the other posts before but now I do. I have to agree that moving the rack and pinion to the rear would be better idea, that way I will have less force on the motor and gear [forward], this could put extra unneeded strain on the motor and gear. Thanks for the idea

The second part I can see being good also, should I just Mount a gear to the shaft/axle and the same gear to the Motor/gearbox and drive that? I could also just use 1 motor, but then I would have more torsional stress on that axle. (I'm really typing my what I'm thinking right now).

I am going to stick with the Rack and Pinion style for THIS elevator. I like it and it seems very useful.

Great Tips so far! Very helpful!

[AdamHeard]

Aaron.

Our gear was aluminum that was waterjetted, and our gearboxes up top were custom and very light.

In our initial conversation I overlooked these differences, that really starts to affect the CG more substantially. Really depends on the game, what you're lifting, etc...

You should do the due diligence and explore other gearing methods as well. Driving the initial stage with chain or timing belt are both pretty easy. The rack gear worked great for us, but we took advantage of the waterjet rack and custom gearboxes to really make it work.

Remember, to model the cascade as single stage, the effective "weight" (in quotes as this isn't actually weight, but close enough for simple models) is = 2*(carriage weight - carriage spring force) + (2nd stage weight - 2nd stage spring force).

If geared reasonably slow for 2 550s, the 1.5 second or so range we talked about, acceleration is effectively instant and you can just look at the loaded speed of the motor and assume it travels at that speed the entire time (as John's calc does I believe). For the faster case, that .22 second range, the acceleration time does factor in more I imagine.

Madison, Jared and Bill.

We found that our bearing setup was nice enough such that misalignment was a nonissue. IF you LOCKED one motor shaft and ran the other full stall, the elevator would kick very slightly and stay in place (Still rolling smoothly, just unable to move due to the motor being locked). We had absolutely no issues with them keeping sync due to this. We tested pretty much all possible combinations; a motor unplugged, running a single motor only, running one motor arbitrarily slower than the PID loop commanded to create a power difference at all times. Nothing caused failure, it just worked.

I wouldn't say this as a blanket statement for all rack and pinion, but our lateral bearing support was able to handle this.

Linking them with a shaft to solve the problem requires a shaft large enough and stiff enough to have enough torsional rigidity due to the length of that shaft.

[Ether]

Originally Posted by akoscielski3

First off I never mentioned putting constant force springs on the elevator to eliminate almost all of the weight.

A loaded spring provides an extra source of power and changes the calculations.

the 28lbs included the outside rail, which is not being lifted at all

If the center of mass of a 28-lb assembly changes from 31" to 57" (a difference of 26"), it doesn't matter what parts move and what parts don't: it still takes 28*(26/12) foot-pounds of work. If you lift the whole assembly 26", or if you only lift half the assembly 52", it's still the same amount of work.

the mass that is being moved would be almost 0, if not positive from the springs.

The springs don't make the mass zero. They just balance the load and provide an additional source of power. The mass still has inertia.