Mounting holes on 84-tooth gear

I was wondering about using the other mounting holes on the 84 tooth gear. The 84 tooth gear is going to be for our arm and have our manipulator attached to it. Has anyone had any problems (gears skipping, gears stripping, etc.) with mounting there manipulators or any metal for there arm on the outer mounting holes? Usually we mount our manipulator to the center mounting holes, but we want to raise our arm higher. Were asking because the big gear makes our arm 17.9 inches tall and we want to raise a little higher to make scoring on the 30 easier. So did anyone have any issues with not mounting there manipulator in the middle of the gear.

We have only centered it on the gear. I would suspect that there would be a little more chance of sligage, but I am not 100% sure.

Shouldn’t be a problem. If your screws are tight, the axle passing through your arm doesn’t contribute to your mount’s strength. I’ve done it before multiple times, and it worked out fine.

That’s perfectly fine.

You’ll have less screws holding the arm to the gear, so they may loosen faster, but as long as they’re tight it should be fine. The torque is the same either way.

If you are really worried about gear slipping, then you could always double up your 84 tooth gears (screw two together) so they are more robust like the high strength gears of other sizes (12,36,60)

we did something similar for clean sweep (so our robot could lock up the green balls) we mounted a piece of metal to the gear (through the middle) and used standoffs to connect it to our main arm 3 inches further up, so in principal its the same concept. i do not recall any misalignment issues but i would double the 84 tooth gear (like Hason said) and use a HS gear (like the metal 12 tooth) to connect it to to have a more durable gearing set up to prevent any issues

There generally shouldn’t be a problem with mounting metal onto the non-diameter holes of the 84 tooth gear. Just remember to place spacers (quarter inch or so) on the screws between arm and gear in order to have better support/less chance of cracking your gears when tightening screws. The torque on the arm/gears shouldn’t be significantly different.

I remember in clean sweep, we mounted each side’s arms to 60 inch gears (narrow non-HS type) and powered them from below with 12 tooth metal gears, two motors to a side. Never found a need to double the gears up, but we did crack a couple of the 60 inch gears (they have really thin material at the mounting holes) at first due to over-tightening of the mounts. Now we only use HS gears.

Note that the length of the arm (in a 4-bar for example), is from pivot to pivot, so moving the metal of the arm to be elsewhere than on that direct path doesn’t change the length of the arm.

It would be a great addition to your engineering notebook to have pictures of the old arm mounting, with a tape measure of how high it can reach,
and then the new arm mounting, and a tape measure of how high it can reach.

I can’t visualize a case where it would help.

we are also using 84 tooth gears on one of our robots, and we mounted the lift bar above the center of the 84 tooth gear (top part of the gear).

By mounting the metal of the arm further from the center of the gear (not through the center of the gear), the arm is becoming longer (assuming they leave the mounting on the other end of the arm in the same place).

And also, by mounting the arm offset from the center-line of a large gear, you will get nominally more height at certain parts of your arm’s rotation. For example, if you had a clean sweep pivot type design, in the “down” position (let’s say angled 45 degrees downwards from mount), with the arm mount offset by 2 holes or 1 inch (offset “downwards” in a direction perpendicular from your arm angle, and you rotated your arm 180 degrees to a dumping position, the mounting location on your arm will be correspondingly higher by around 0.7 inches (effectively a leg of a 45-45-90 triangle with 1 inch as the hypotenuse, so not very much). Correspondingly, only rotating the arm 90 degrees, to an “upward” 45 degree inclination (more applicable to gateway), with the same starting perpendicular offset of two holes on the gear, will shift your mechanism forward by that 0.7 inch amount.

The small effect of this mounting can be increased by a method similar to that described by Paul T. 27. You can use whatever distance you like by offsetting your main arm by use of standoff or structural attachment to an arm element on the gear’s center-line.

Does this make sense, or would a diagram help?

However, if you are doing this to maximize arm reach, I would advocate rethinking your arm gearing. Perhaps use compound gears so you can use smaller driven gears (attached to the arm) that don’t include so much dead space at the [presumably] top back corner of your robot.

Right now, we’re working on a pivot type design similar to Elevation bots, but a 4-bar linkage driven with a 1/2 size bottom bar, so that the top bar or pivot arm’s pivot point can be at 17.5 inches without worry of the 18 inch limit. This also allows us to mount our motors lower for nominally lower center of gravity and little need for extension cables. The bottom driven pivot point is also shifted forwards in order to have maximize the top pivot point’s position at the very back corner of the robot and have the gearing clear the back of the robot. This 4-bar link is funny looking but actually extremely close to a parallelogram.

A diagram would be nice. I am lost with what paul t and you are saying.

Here’s a picture that clarifies what I’m saying. VEX Forums re-sized my wonderfully and painstakingly crafted MS paint diagram, but it is still somewhat readable.

To sum it up:
I.) If you have maximized your arm length already as the diagonal of 18 by 18 size limit from side view, you will not get any more reach at the end of your arm by offsetting the arm attachment point on your gear. Effectively, your pivot point and arm endpoint will still be the same no matter how you offset.

II.) There are alternate reasons to do this however, including (in clean sweep) having the arm mount point end up higher at certain parts of rotation. Also, you will be able to minutely tweak the length of your arm rails in order possibly hold more objects.

III.) If just looking for ways to increase the reach of your arm, you will have to eliminate “dead space” of pivot point not being at height limit of robot. This is usually caused by the size of your attached gear or sprocket. Options include:
[INDENT]-Use smaller gears for attachment to your arm. This usually means you have to use a compound gear ratio in order to get the correct reduction. As in motor driving 12 tooth which drives a 36 tooth, with another 12 tooth on the same axle, which drives a 36 tooth etc…

-For pivot, 4-bar, 6-bar type arms, you can consider only powering (gear attached to) the bottom bar (attach pivot to a 4 bar) This allows you to have the top bar’s pivot point (the one that actually matters for simple pivots arms and 6 bar links), to be alone at the very top of your robot (~18 inches).

-Similarly, in order to further maximize the length of the bot’s arms (imagine them as the hypotenuse of a triangle with the legs being the robot’s length and height), you want to avoid having dead space behind the robot due to the gear or sprocket sticking out. [EDIT: Move] the powered bottom bar’s pivot point forward in order to clear the back edge of your robot. Then compensate for the shift and approximate a parallelogram with the 4-bar linkage.[/INDENT]

No such thing as magically farther arm reach, but these suggestions can optimize the reach of pivots and 6 bar links by an OK amount (1-3 inches). If the top pivot point is at 17.5 inches, and the arm is “stowed”, touching the ground in a 45 degree downwards angle, when it is swung upwards to a 45 degrees upwards inclination, the very end of the arm can in theory reach 35 inches (17.5" +17.5") . And, with the arm nearly 25 inches long, at the top of it’s arc, vertically above the robot, it would hypothetically reach 42.5 inches (17.5"+25"). It’ll help you clear the tops of all the goals without straining for reach. You also get the benefit of having a lower position for arm motors, useful given that most Gateway robots seem to have pretty high CoG. (Especially the object filled six-bar bots)
explanation of offset gear mounting and alternative ways to increase reach.jpg

Nice explanation, Edward.

One thing I would emphasize is having your arm the full 25". Moving where the arm mounts to the gear to be further forward will get you part of the way there, but only a few inches at most.
The arm length probably matters the most here. As long as your pivot is somewhere above 15" (which is really easy to do), you can get your mechanism moved up to 30" and be directly above where it started on the ground. Moving the pivot to above 15" lets you reach 30" such that the mechanism is further forward than where it started on the ground. Increasing the arm length increases how much further forward you can get.

My team tried this with a conveyor belt for the first competition this year. It shouldn’t be a problem. However, the gear may be slightly tilted depending on the weight of your arm. If you feel insecure about it (like us), screw two together and use it like a high strength gear.