Reducing Gear Slop

Hello everyone! I’ve been trying to fine-tune my team’s six-bar recently, and have encountered some problems related to what I would define as gear slop (unwanted movement by gears before they encounter resistance from the motors). Our lift has up to 0.5" of unwanted vertical movement due to this slop. Any helpful advice or pertinent links would be appreciated.

Some background information: our six-bar is using a 15:1 compound gear ratio, 3:1 driving 5:1.

Many thanks, IGC Alinator


Our team (4886a) also runes a 1:15 for torque compound gear ratio on our lift and the slop is there just because of the compound gearing. (At least that’s what we’ve concluded.)


What kind of support do you have between the 5:1 and 3:1? You could add a lock bar in between to reduce the slop a bit but it probably won’t get completely eliminated. Another option would be to just take down your ratio to 5: or 7:1, unless you are planning to lift 25 sacks at a time.

I’m not completely sure why would you want to remove that. I not completely sure, but i believe that acts as a safety system. Your motors would spin slightly, then face greater resistance rather than great resistance immediately, creating a very slight curve. Also, i would think that it allows you to manipulate your robot to fit within dimensions easily because you can adjust the arm slightly.

But if you do want to remove it, maybe you can use 24c’s method of apply lock plates to the 3:1 ratio and maybe a trimmed down 1x25 bar between the 5:1 ratio. If you want to be a bit complex, i think planetary gearing can reduce slop

We call this wiggle room and it’s undesirable because it’s difficult to program or drive with precision. With wiggle room when you hold up on your joystick or program your arm to lift for 3 seconds, the motor turns for a little bit before the wiggle room is used up, the axle locks and the arm actually begins turning.

I like SweetMochi’s terminology here. The “wiggle room” is caused by the axles that your lift gears are mounted on being able to move back and forward a little, even with tight bearings. This means the gear that the arm is attached on to can spin a little even if the motors don’t turn.

In the case of compound gearing, there is a higher amount of slop encountered because there are simply more shafts in the lifting system. This means there is more total back and forward movement in the gears, so you end up with a larger amount of slop. One way I have found to reduce this is to replace some of the axles used on the lift with bolts. The centres of high strength gears can be removed and replaced with circular inserts that the bolts fit inside. The bolts allow less back and forward movement, so the wiggle is slightly reduced. However, I still think despite any precautions taken, a fair amount of wiggle room will always be encountered in compound gearing systems.

Hmm, that is an interesting idea, thanks for the input.

It’s called backlash and is pretty much unavoidable with the parts we are using.


Power the arm up and down (with sacks on the intake) and observe the compound gearing very closely. Some of the “slop” will be due to the axles flexing, before they start turning, as load is applied. To minimise this axle flexing, design your towers so the axles are as short as is physically possible (or even shorter :slight_smile: ).

Cheers, Paul.

I know this thread is about gears, but I’ve found using sprockets and chain will tend to reduce a lot of slop. Originally we used compound gearing, which had the slop, but after switching to a sprocket/gear compound the slop appeared to be nearly all gone. Plus the chain can act as a safety net where it’ll more likely snap before ruining the motors.

As for the slop in the gearing, I don’t think there is any real way to prevent it with the vex gears.

Interesting, we’ve found the exact opposite due to extra slop from any looseness in the chain. Unless you use the chain and sprockets to avoid compound gearing I feel like it would have worse performance.

1 Like

I put a compound gear together today, the same 8.33:1 as the students are using. The backlash is not really that bad, about 1/2 tooth of movement on the 60 tooth final gear. The problem is 1/2 tooth is about 3 deg of movement. Lets say you had a 15 inch arm on this, the movement at the end of the arm would be (approximately).

movement = 15 * sin(3)
which is 0.79 inches.

or the correct way, ( 3.141592 * 30 / (360/3) )

Add in some wear for old gears and perhaps sloppy construction and it will only get worse.

I’ve found using tensioners, like a nylon spacer, can set just the right about of slop. The only problem being that sometimes the chain can jump off the sprocket and skip a tooth when under a large load, but I suppose gears can do the same.

Yes, tensioning can help, but chain is more liable to jump than gears. When there’s a large load and the arm motors try to turn the sprocket, tension builds up on one side of the chain while tension loosens on the other. The chain tends to break on the tense side and skip on the loose side. Since breaking is worse than skipping, I like to design a bit of looseness to avoid breaking which ends in a bit more slop than with gears.

Fair enough. I guess I need to experiment more =P