I am trying to make a lifter similar to sewer snake for a 30lb robot. I am not trying to throw someone to the roof, but just to lift them as I push them to hit them against the walls. I am trying to get a reasonably fast reacting lifter. Could I use maybe two magnum 550 motors (I think they are 20:1 ratio gearboxes) directly driving a single pivot lifter. If not could you guys point me in a direction of what motor might help me accomplish this design. Thanks.

Hi.
I would first start with a 16mm ball screw they work with very min friction and arnt to expencive off ebay,another point would be to look at a 4 bar lifting set up they are a little harder then a single point but well worth it.
you could also use an electric linear actuator but they are abit pricey.
In my last bot (which I still havent finished yet ) I used a 3 start m16x2 threaded rod with a 14.4volt xu1 drill motor/gearbox and a small bearing support for the end on the rod and it was pretty quick I just need to change the 4bar set up a little becasue the motor was using to much power to move the steep angle,but other then that worked pretty good and didnt cost much.
hope that helped alittle

Sewersnake's design is a classic, I have been looking at a lifter more like Pipe Wench for some time. Those magnum motors might be physically tough enough, but the gearing is probably too high.

Here is my reasoning:

* The lifter arm needs to travel much less than 360 Deg. for a lift; I picked 90 degrees for the total arm motion.

* The lifter arm doesn't need to move all that fast, I figured .25 sec end to end was fast enough and a convenient number to work with.

* 0.25 sec for 90 degrees of movement works out to 1 rps or 60 rpm (I did say it was a convenient number! )

* The Magnum gearboxes have an output speed of 900 rpm @18v, so they would give you 15 rps or a lift time of .017 seconds; too fast IMHO.

* Now you need to look at the torque the gearboxes develop, a theoretical 17.6Nm - lets round it down to 15Nm (30Nm for two motors) after friction losses and over enthusiastic manufacturer specs. The bot needs to lift the opponent and assuming even weight distribution and that the other side of the bot is still on the ground, the lifter needs to to get 15 Lb moved a distance defined by the length of the lifter arm in a time defined by the gearbox speed.

This is where my engineering knowledge fails; I am not sure what formulas to use to use to combine the rpm, arm length etc into a torque value for the gearbox - if anyone who actually took an engineering course would like to step in here, that would be appreciated!

What I was looking at was a single Banebots P90 gearbox with a 4 stage reduction and an extra 2:1 reduction coupling to the lifter arm. I never picked a motor due to the problem with the maths - too big a motor wastes weight and too small a motor won't perform.

Thanks for the help guys.

Nick, here are some formulas and explanation of TAF that I was given by Aaron and Mark J. when I posted a question on the Ask Aaron website. I don't know if these will help you at all.

Optimum Lifter Gear Reduction = (Arm Length [inches] * Lift Weight [ounces] * 2) / Motor Stall Torque [oz-in]

The other formula I was given is:

Lift Capacity [pounds] = (Motor Stall Torque [oz-in] * Gear Reduction) / (Arm Length [inches] * 16 oz/lb * 'TAF')
Peak Amps @ Lift Capacity = Motor Stall Current [amps] / 'TAF'

Peak Amps @ X Pounds Lift = Peak Amps @ Lift Capacity * (X pounds / Lift Capacity)

This is the explanation Aaron gave me of TAF.

What is 'TAF' in the equations? The more heavilly a motor is loaded, the slower it runs. If loaded to its full stall torque rating, it will not move at all. The 'Torque Allowance Factor' at the end of the formula de-rates the max load to reduce current draw and allow a reasonable lift speed. I use a very conservative allowance of 2 for my robots. Some builders leave the allowance off entirely (= 1). You'll be OK with a 'TAF' around 1.5.

Interesting, thanks for reminding me about that 'Ask Aaron' page I had not visited for years. I think Aaron left out speed & time in those formulas - you could make almost any motor and reduction work, but you also need to factor in the tip speed of the lifter arm - too much requires a large & heavy motor, while too slow allows the opponent to escape and you lose control.

Matt Maxham is a very friendly & helpful guy; I just emailed about him about how fast Sewersnake's arm moves (IE the gearbox output RPM) and we can work that back into the equations.

That's what I was thinking too about the speed and time. That's what I was really trying to get an answer about. I just don't want it to lift so slowly that the other robot is able to escape. Thanks again for the help and emailing Max Maxham.

Thanks again for those equations, they are starting to make your proposed motors look quite plausible. For 2 motors at 14.4V and an 18" arm, I get a gear reduction of 31.08, which would be lower if you were running at 18V but I don't have a value for motor torque at that voltage. Even if you ran at the lower voltage, you would only need a further 1:1.6 reduction to satisfy the formula, which would be easy to do with a chain or gear final stage.

Just a thought; how about running a dewalt 36V hammerdrill & gearbox thru the equations? I think they will need an extra reduction stage but might be better value and performance. I impulse bought one awhile back and its an absolute beast and very sturdy

In that 2nd equation, what does this bit mean: "(Arm Length [inches] * 16 oz/lb" I am not sure about that oz/lb part - perhaps I am only use to thinking in metric

All the maths is above me, but one of the biggest issues with such big reductions going into lifter arms is all it takes is a solid spinner hit to break one tooth/pin whatever and you're entire mechanism Jams up. SJ is another very impressive heavyweight lifter and not many people know but it had an ingenious actuated front wedge, with many high reduction gearmotors and chains. First hit versus a very high power spinner broke *every chain* instantly.



I've really wanted to try a hypocycloid gear box one of these days, they wouldbe hands down the most robust solution to getting a huge reduction of speed and increase in torque in a compact area. You will however need a pretty good CNC to produce the gearbox custom.

http://www.youtube.com/watch?v=cnJCWX2nr4M

That's my left wing input.

I think SJ's design is just asking for trouble - mounting motors to the wedge will crack their magnets on a good spinner hit and it looks like they are using worm gear reduction on the first stage, which is not very efficient. Breaking all six chains is pretty impressive though, it must have been a spectacular hit!

I saw in the first of Aaron's equations that reducing the lifter arm length to 12" put the gearing down to 20.7, so the dual Magnum motors are looking good and my higher reduction theory is out the door - now we just need for find the sweet-spot for arm speed - another reason for not having to too fast is that you might end up flinging the opponent off the arm (IE a very weak flipper) and that would mean less control points.

General question: if I know the Torque constant of a motor and it's stall amps, will multiplying them give me the stall torque? If that's the case, I am really liking the look of the new style 36V Dewalt hammerdrill with it's gearbox set in the lowest gear!

Here is an example done with a PDX 256 motor they did. I think they are just multiplying by oz since the motor specs are in oz. 16oz in one lb. Yeah we're all still lbs and oz here in America. We have to catch up to the rest of the world. The lifter arm in the example was 8 inch lifter arm to lift 30lb robot.

Example for PDX256 gearmotor:

Lift Capacity = (91.6 oz-in * 256) / (8 inches * 16 oz/lb * 2) = 91.6 pounds
Peak Amps @ Lift Capacity = 148 amps / 2 = 74 amps

Peak Amps @ 30 Pound Lift = 74 amps * (30 lb / 90 lb) = 24.7 amps

Here is another formula:
Stall Torque (oz-in) = length of lifter arm (inches) * weight class (ounces) * 1.67

Here is some info on Sewer Snake that Max sent to the Ask Aaron website:
"The fork/wedge is powered by a Mini AmpFlow motor on a 25:1 Apex gearbox to a 3:1 chain reduction. The chain is attached to the weapon mount so the mount can spin freely 360 deg. There are no electronic stops for this weapon system, I have to remember when to turn off the power or reverse direction so I don't burn up the motor!!! Weapons can be designed and mounted in any position [on the weapon mount]. With the red forks the receiver is in the up and angled down position. With one of the wedges for spinners we have the receiver down to keep the angle of the wedge as low as possible!
The [red] lifter fork on top of the robot works in conjunction with the wedge/fork on the front. The lifter fork is mounted on a pin and only gravity holds it down, but when the wedge/fork is rotated forward it pushes the lifter fork up. When the wedge/fork is rotated backwards it lets the lifter fork fall back to a resting position on the top of SS."

OK, so now we know about how fast the arm rotates on SS: 6,000 rpm for the short Mag / 75:1 reduction = 80 rpm. That's just a bit higher than my theory, I don't feel so dumb now and under load SS's arm would rotate a bit slower anyway. Now we can reverse the equations and find a motor/gearbox combo that will do the lifting at the target speed.

Its hard to recall a forum discussion that has been so productive and interesting as this one. Perhaps we can turn this into a cookbook for new builders, it would be great to have some effective and entertaining lifters on the circuit.

I reckon you wants a harmonic drive, stick that in yer CnC ;-P
http://www.youtube.com/watch?v=q1v7a0jsp1I
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Mechanical engineers build weapons, civil engineers build targets

I think Chris Baron was investigating a harmonic drive gearbox and discovered that it really wouldn't take shock loads common in combat bots. I'd still like to see someone try though; if it worked there would be some very cool lifters in the competition.

As much as I love the harmonic design cost benefit in the high shock operating environment does not seam worth it. Should state I have only used one once and studied them for a weeks worth of class so Could easy be wrong.
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Remember to trust me, I am an Engineer.

http://www.youtube.com/watch?v=rp8hvyjZWHs