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Unread 12-17-2012, 09:53 PM
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Elevator Design - Rack and Pinion Cascaded - 772

My First Elevator System

- Currently 28 lbs
- Fully extended in ~1 Second (possible to raise in 0.22 seconds but i geared it down)
- Run off of two 550 Series Motors into a VersaPlanetary 2 stage gearbox (1:20)
- Rack and pinion style for bottom
- Carriage is attached to a fixed Belt or chain that will move the Carriage up to the top when the middle moves up. (hard to explain, but no motors required)
- Rail design Inspired by 228's 2011 Elevator Design.

Design later found to be almost the same as 973's off season Robot in 2011 "Emperor Swerve"

Feel free to criticize, and suggest things to change.
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Last edited by akoscielski3 : 12-17-2012 at 10:00 PM.
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Unread 12-17-2012, 09:59 PM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

Posted Pictures rather than having to Download PDF's. Sorry for double post.





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Unread 12-17-2012, 10:22 PM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

Hey there,

Here's some basic questions to start you off:

-Is this the lightest way you can build an elevator?
-Is this the least resource intensive way for your team to build an elevator?
-Why is a rack and pinion setup better then a string and drum?
-Will the channel of the middle stage deform being pushed on from the inside and outside by the inner stage and the rack and pinion?
-Is putting the motors up high on the elevator really the right place?
-Will your roller brackets deform over time or when the inner/middle stage takes an impact?
-Is using the motor as the front top middle stage container really a very good idea? If so could the motor be moved higher to improve your lever arm in the front containing the middle stage?
-How is this going to be attached to the actual robot?
-Is the single round crosspiece on the middle stage enough to keep it torsionally stiff?
-Do you have a plan to closely control the motor and pinion relative to the rack? (spacing wise?)

Looks pretty good overall.
Regards, Bryan
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Unread 12-18-2012, 07:42 AM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

-Is this the lightest way you can build an elevator?
I have some thoughts about making everything lighter, first off would be getting smaller bearings. But the rack and pinion style cascaded does seem lighter than the others because I don't have chain or require a bunch of pulleys.

-Is this the least resource intensive way for your team to build an elevator?
I did keep resources in mind while designing, for example most of it has angle iron rather than water-jet plates. But all of these parts are possible to make with our resources. I want remove some parts that are only there for support or spacing, but we'll see.

-Why is a rack and pinion setup better then a string and drum?
The first reason I dis it this way was because of a meeting I had with my designing team (Mechanical Section). On of my members suggested a rack and pinion rather than using the strings or chain. It was really a "shot in the dark" but I like how it is.
The second reason would be simplicity. Though some people will argue how simple this is, i believe it ia a simplier set up than a string and drum. This way the only moving parts is the gear on the rack, and the carraige (moved with belts), rather then having string or chain all through it.

-Will the channel of the middle stage deform being pushed on from the inside and outside by the inner stage and the rack and pinion?
Something I never really thought about. I will be doing stress analysis soon on it to find out.

-Is putting the motors up high on the elevator really the right place?
There is no other logical place to put them. If i had them at the bottom the gear would run off the pinion almost emediatly, and if i were to put it on the inside rail it would make me have to put more wires and there would then be more moving wires, which can get annoying.

-Will your roller brackets deform over time or when the inner/middle stage takes an impact?
Another thing I never thought of. Again I will do stress analysis on it soon. However I don't believe it will be an issue. Because there is so much contact space between the bearing set up and the bracket it will be tought to deform. Unless you're talking about where it mounts on the rail, and in that case it may be able to deform, but i dint think by a lot.

-Is using the motor as the front top middle stage container really a very good idea? If so could the motor be moved higher to improve your lever arm in the front containing the middle stage?
I think it's fine, but i am thinking of putting some bearings on there also. Im not sure what you mean for the second part.

-How is this going to be attached to the actual robot?
I'm going to add brackets to the back to help stabalize it and mounts to the bottom to mount onto a chassis.

-Is the single round crosspiece on the middle stage enough to keep it torsionally stiff?
Wasn't thinking of torssion. That brace was for the pulley, but now i realize i needto add strength there.

-Do you have a plan to closely control the motor and pinion relative to the rack? (spacing wise?)
I have it slotted at this moment, but should I consider doing something more??

Thanks for the questions, it really got me thinking about it
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Unread 12-18-2012, 07:52 AM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

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Originally Posted by akoscielski3 View Post
Currently 28 lbs... Fully extended in ~1 Second (possible to raise in 0.22 seconds but i geared it down)
Can you say approximately where is the center of mass in the the fully retracted and fully extended positions?


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Unread 12-18-2012, 08:01 AM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

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Originally Posted by Ether View Post
Can you say approximately where is the center of mass in the the fully retracted and fully extended positions?


I can edit this to tell you the exact in about in hour. But if i remember correctly its 12" when down, 22" when fully extended.

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

When it has a 40lb chassis mounted to the bottom (just mated, not having mounting brackets or anything) the center of gravity is 11" from bottom of elevator (elevator down) , and 22" when Extended.
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Unread 12-18-2012, 10:14 AM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

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Originally Posted by akoscielski3 View Post
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 ?


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Unread 12-18-2012, 10:52 AM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

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.
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Unread 12-18-2012, 11:04 AM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

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Originally Posted by Madison View Post
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.
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Unread 12-18-2012, 11:05 AM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

Quote:
Originally Posted by Madison View Post
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.
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Re: Elevator Design - Rack and Pinion Cascaded - 772

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?
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Unread 12-18-2012, 11:31 AM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

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!
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Unread 12-18-2012, 11:46 AM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

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.
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Unread 12-18-2012, 02:19 PM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

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Originally Posted by akoscielski3 View Post
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.

Quote:
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.

Quote:
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.


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Unread 12-18-2012, 09:08 PM
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Re: Elevator Design - Rack and Pinion Cascaded - 772

Ok, I have a little more time now to type out my thoughts in a little more detail.

-Is this the lightest way you can build an elevator?
I have some thoughts about making everything lighter, first off would be getting smaller bearings. But the rack and pinion style cascaded does seem lighter than the others because I don't have chain or require a bunch of pulleys.
-A rack and Pinion setup is not lighter then a well thought out solution using spectra cable (high strength kite string) and pulleys. Even if it wasn't the rack and pinion forces you to put all of your motors/reduction at the very top of the robot (bad) and requires much higher precision (meshing gears) then a string design does (generally aligning pullies.)

-Is this the least resource intensive way for your team to build an elevator?
I did keep resources in mind while designing, for example most of it has angle iron rather than water-jet plates. But all of these parts are possible to make with our resources. I want remove some parts that are only there for support or spacing, but we'll see.
-Allowing the Grounded stage's width to be adjustable is a very good idea. It's very unlikely that you're stages will match up as perfectly as in CAD so leaving room for adjustment there can make a big difference between good and bad.

-Why is a rack and pinion setup better than a string and drum?
The first reason I dis it this way was because of a meeting I had with my designing team (Mechanical Section). One of my members suggested a rack and pinion rather than using the strings or chain. It was really a "shot in the dark" but I like how it is.
The second reason would be simplicity. Though some people will argue how simple this is, i believe it ia a simplier set up than a string and drum. This way the only moving parts is the gear on the rack, and the carraige (moved with belts), rather then having string or chain all through it.

-You kind of make it sound like you didn't really think through your elevator options and just picked one. Do you have any reasoning aside from personal preference? I'm not trying to be mean or anything, just trying to better understand your reasoning.

-Will the channel of the middle stage deform being pushed on from the inside and outside by the inner stage and the rack and pinion?
Something I never really thought about. I will be doing stress analysis soon on it to find out.
-I would guess that this will occur a little over the course of the season. Maybe just keep an eye on it throughout the season if you actually build this for 2013.

-Is putting the motors up high on the elevator really the right place?
There is no other logical place to put them. If i had them at the bottom the gear would run off the pinion almost emediatly, and if i were to put it on the inside rail it would make me have to put more wires and there would then be more moving wires, which can get annoying.
-I was kind of leading you on with this question. No, there's not anywhere else to put the motors except for at the top. I'm a CoG and weight management freak which really stems from my understanding of how it affects drivetrain performance. I pretty much dislike any design in which the motors, gearboxes, and other heavy stuff isn't all the way at the very bottom. It's a primary reason I would never build an elevator this way. Of course, your robot, your preference.

-Will your roller brackets deform over time or when the inner/middle stage takes an impact?
Another thing I never thought of. Again I will do stress analysis on it soon. However I don't believe it will be an issue. Because there is so much contact space between the bearing set up and the bracket it will be tought to deform. Unless you're talking about where it mounts on the rail, and in that case it may be able to deform, but i dint think by a lot.
-These are actually very concerning to me in this design. Cantilevering those rollers through only a 1/8" piece of metal is simply not enough. I bet if you put the elevator all the way up and pulled on the top you could bend those brackets right out. These defiantly need to be strengthened or I guarantee that they'll fail at some point in the season.

-Is using the motor as the front top middle stage container really a very good idea? If so could the motor be moved higher to improve your lever arm in the front containing the middle stage?
I think it's fine, but i am thinking of putting some bearings on there also. Im not sure what you mean for the second part.
You have to think about lever arms. When the middle stage is all the way up and you pull on the top it needs to be supported at two points in order to not come out of the Grounded stage. The further apart these two points are the better your lever becomes such that the pulling at the top exerts a lesser force on the bottom then it otherwise would. To maximize this you would want to put the contact points from the Grounded stage to the Middle stage as high as possible and vice versa for the middle stage.

-How is this going to be attached to the actual robot?
I'm going to add brackets to the back to help stabalize it and mounts to the bottom to mount onto a chassis.
-If the elevator wobbles at all very bad things will happen. Whatever you do make sure the bottom is very well secured.

-Is the single round crosspiece on the middle stage enough to keep it torsionally stiff?
Wasn't thinking of torssion. That brace was for the pulley, but now i realize i needto add strength there.
-The Middle stage needs much more support. If the middle stage can twist, besides introducing slop into your rack and pinion you end up with a big wobbly thing up in the air and potential binding. Maybe try to get a 2x1 1/8" thick box tube across the top and bottom of the middle stage.

-Do you have a plan to closely control the motor and pinion relative to the rack? (spacing wise?)
I have it slotted at this moment, but should I consider doing something more??
-I would guess that if your hand making the brackets supporting the motor this could be a problem. Slotting the holes seems like a good solution to me.

Thanks for the questions, it really got me thinking about it

-That's what design reviews are for! I just went through the 3rd review of a sheet metal and thinwall Aluminum tubing elevator. At the first revision it had a bunch of complicated folds and extra material. By the third it weights 14lbs and has been optimized for ease of manufacturing and assembly for my team. Every time you go through the process you make your product a little better until you end up with something that's awesome.

Keep up the good work!
Regards, Bryan
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