My team has decided to use a scissor lift this season. We built it more or less successfully, but we noticed a few issues when we used it in competition.
I have created two galleries of images. The first one only includes photos of our scissor lift from today. The second one includes photos taken over the past 1-3 weeks, but may offer a better/cleaner view of the scissor lift.
The scissor lift is powered by a vertical rack and pinion system using two 393 motors on each side.
The main problem we’re having is that our scissor has seemingly inconsistent lifting ability. Sometimes it works near perfectly, but other times we have trouble getting it to operate at all (sometimes it just seems to get stuck). And on occasion, it happens to one side of the lift at a time.
Another issue we’ve been having is that the two sides of the scissor lift become misaligned (I suspect this may be related to the problem above). In many cases, the difference is negligible (and I would suspect this is fairly normal for standard scissor lift operation). However, sometimes the two sides go very askew. It seems as though, when operating the scissor lift, one side of the lift inexplicably lags behind.
The other issue we’ve been having is that the vertical racks break. We used to have an issue with the plastic pinions as well, but switching to the metal alternative seemed to resolve that. However, the teeth of the racks still seem to break off periodically. Sometimes this doesn’t affect our overall lifting ability, but in a few cases it essentially breaks our lift and we are forced to replace the rack. We’ve already gone through a fair number of racks.
Unfortunately, we don’t know what exactly causes these problems, and we don’t really know how to resolve them. I am looking for advice on how we can make our scissor lift better, and hopefully fix some of the issues we’ve been experiencing.](Robot - Album on Imgur)
Correct me if I’m mistaken, but I can’t see any sensors on the lift. If you add some of those then you can implement programming so that if one side is lagging, slow down the faster side until the slower side catches up. In my experience with scissor lifts (and other lifts) the more difference you get between the two sides, the greater the friction and the worse the problem gets. If you use that sensor feedback then you can keep the offset and friction to a minimum. My preference for sensors would probably be encoders on the motors, also with limit switches to prevent lifting too high or too low.
I made a prototype with this type of vertical rack and pinion system and I really liked it because of its constant lifting speed/torque no matter what the height. But all scissor lifts seem to be prone to locking up, mainly in the sliders at top and bottom. The best you can do is keep everything tight and as straight as possible and use legal lubricants if you’re not already doing so. I have found that once the mechanism has been out of alignment a couple of times it gets more and more prone to happening again, just because things are slightly bent and it is hard to identify one cause.
I am not too surprised that you’re breaking racks, because you are putting a lot of motor power through them. Those racks came out when we were still using 3-wire motors so probably noone ever expected two 393s to operate on each one! One simple idea is to use a separate pinion for each motor, one above the other so the load is spread across 2 pinions instead of just one.
Out of curiosity, how much weight is this thing lifting?
One thing I have to say is that the linear slide bracket, the part where the motor and steel pinion gear are attached, does have a problem where they flange apart or inwards just a little bit, but enough to cause extra friction as well as possibly allowing the steel gear to move away from the rack, so you have to use a standoff and two-three steel washers to keep them the same distance apart. I built a 1:1 ratio of that but I found it worked very smoothly under no load. But I did need more torque so I went to 13:6 ratio, just mentioning it in case you might want to reconsider your ratio. My scissor lift is using the plastic sliders and I see that you have the metal ones but I’m not sure if that is a problem. I spray silicon lubricant on the sliders as well, I recommend that because it makes a world of a difference. I have also never had a problem with the rack breaking, and I once had 4 393’s at a 3:1 ratio on stall torque and they still were fine. The only breakages I’ve had is the square hole of the sprocket round out under all that torque.
We do not currently have any sensors on our lift, but we are planning on attaching integrated motor encoders soon (we just haven’t had time yet). We’ll definitely implement corrective programming once we get those attached, but we were hoping that the error would be relatively small and that we’d be able to have at least a somewhat functional lift today. It seemed to become very significantly misaligned rather quickly - as though one side of the lift just suddenly cut out. If I recall correctly, at one point, one side of the lift was at nearly full height/extension, while the other was barely halfway up.
We tried our best to lubricate and keep everything as straight and tight as possible, but we were still experiencing problems. Do you have any recommendations as to how we could either change our design or somehow improve our straightening/tightening/lubricating process?
We’ll definitely give 2 pinions a try.
Also, I forgot to mention this earlier, but another issue we were having was that the lift would tilt/sway from side to side (especially when fully extended/raised and driving/strafing sideways). Most of the time it isn’t too bad, but I’m concerned that during a match we could tilt over or break our lift if our lift is at maximum height and we quickly move sideways. Is there any way we can fix this swaying motion?
Our team has also been building a scissor lift for the first time this year, and we’ve gone through a ton of different designs. As far as breaking teeth goes, I too would recommend using two pinions. That was a common problem for our initial design which was powered by horizontal rack and pinions. Make sure your pinion gear is making good solid contact with the slider teeth; we had to add some washers under the rack to bring it closer to the pinion. Not sure if that’s so easy in your design, though . . .
In all, your problems seem to be originating from the fact that you’re overpowering the lift. I would try and reduce weight on the lift wherever possible, cutting out all steel. Rubber bands, which I see your using, will also help. You may also want to consider using a compound gear ratio, but I know that’s kind of a last resort. If all else fails, I’d recommend looking into the planetary / Chinese-style scissor lift, which is able to make better use of the force from your motors.
Swaying was also a problem we had early on, and we were able to almost completely eliminate it with cross braces (as in 3x31 ish c-channels connecting the left side of the scissor to the right side). It looks like you already have one or two cross braces, but I would attach them directly to your scissor and use as many screws as possible (4 rather than 2). We used a total of 3 (one on each stage) each connected with 4 screws. If the cross-braces are the right length and are secured tightly, they should eliminate most of your sway.
An easy fix is only allow robot to drive at half speed when the arm is up. Obviously this is an inefficient fix but remember tipping is inefficient as well. Save the robot then win the match.
A shaft going across between the two motors would work best. The mechanical coupling of the two sides ensures both sides will lift at the same speed.
You might also want to ensure the rubber bands get to opposite sides of the scissor. Standoffs can be a bit easier to control than the wrapping of the rubber band around the end. If you have the room putting rubber bands on the inside and outside helps the scissor from being pulled outwards.
Lastly, ensure there are Teflon washers between every moving part even if there are delrin blocks. Use nylocks and ensure there is a slight bit of wiggle room to minimize friction but still give rigidity of the scissor.
We are doing a scissor lift this year and we had similar problems. Our biggest problem is that we need to have a fully charged battery in. And I’m not sure but could a power expander work for raising and driving at the same time drawing a lot of power at once?
We built a very successful scissor lift for Sack Attack last season. We used gear mechanism, not rack and pinion. This year we built a similar scissor lift.
