# Use Bosch seat motor for folding arm

#1

Can we use the Bosch seat motor in the folding arm? I’m not sure if it’s powerful enough?

#2

You’re going to have to give a better description what “the folding arm” is if you want a reasonable answer

#3

Depends upon what you want the arm to do. We’re using it in our all female rookie team’s Everybot to manipulate hatches. I don’t think it would be bad for other uses as it lifts our arses in a car seat.

The only way you can be sure is to test it. (just call me Capt Obvious this am)

#4
• Stall Torque: 22Nm 16.2 ft-lbs

Have you calculated how much torque you’ll need? A good rule of thumb is to gear the motor to provide double your calculated torque.

#5

You really need to know how much each of the parts of the arm weigh and how far the center of gravity is from your pivot point for each part. From that, you can calculate the torque needed at the pivot to turn the arm. Once you have the torque at the pivot, you can go to the “Rotary Mechanism” tab of the “JVN Calculator” and calculate what motor and gearbox combination would work for you. You will also have to decide how fast you need the arm to rotate. It is likely that the seat motor is not in the database in the calculator. It is also likely that you need a bigger motor like the ones that are in the database. There are several YouTube videos showing how to use this calculator. Consider this part of the STEM education that this program promotes.

Lastly, it would be of great benefit to you and your team if you got in touch with some of the established teams in your area. Some contact details were recommended to one of your team members in this post. There is a lot to learn for a rookie team like yours and the established teams can help you get up all those learning curves much faster. There is no shame in asking for help.

#6

Speaking from experience, I would strongly urge you to not use the Bosch seat motor in any “arm” application, at least not one of any significant size. There are use cases for it, but most arms are decidedly not it.

Torque, speed, and power aside, the Bosch seat motors have an integrated worm gear system made of, iirc, a nylon gear meshing with a metal worm. (Still do the math, though, but for a different motor+gearbox combination, please)

You might think, “oh, it can’t be backdriven! Cool! Let’s use it.” Don’t. First, I would be willing to bet that any somewhat large arm will easily backdrive it, so you get minimal benefit anyway. Second, not being able to backdrive (at least not easily) is another way of saying that all those forces are going straight into those plastic teeth, which are now getting continuously worn down if not broken outright. We did this last year, and it went extremely badly. At least 5 motors were damaged, and I wouldn’t count on anyone having spares at any event you go to. We ultimately had to replace it with a 775pro + Versaplanetary from teams 1058 and 2877 (<3). Still not ideal, especially the way we had it set up, but was far far better than the Bosch motor was.

But wait, there’s more!
The Bosch motor has a female output which requires an adapter to turn it into 1/2" hex (or anything usable). It’s not like other motors, where the small output shafts are never seen outside the gearboxes anyway. You’re likely never putting this thing in an external gearbox (good luck trying), so you need to work directly with what it has. The adapter from AndyMark, while nice, is at times quite frustrating to work with. The female output is also 1/4" square, as I recall, and so the shaft at the point where you need the most strength is… Not quite up to snuff. In case you’re wondering, yes, we have actually twisted these shafts. And if the shaft doesn’t fail, there’s a not insignificant chance that the motor’s female output will. It’s just a cutout in the middle of the last (plastic) gear. Not too difficult to round off.

It also has a weird electrical connection. I’m not an electrical guy, so I can’t comment on the objective merits of the connector, I just know we had to go and get a special adapter to hook it up properly.

Relevant pictures attached:

Oh, and here’s a video of someone messing with the insides. Notice the missing gear teeth? Yeah, it wasn’t the pencil that did that. https://www.chiefdelphi.com/uploads/default/original/3X/d/3/d3e5eef42724433442b17932fcdc204e93ecf721.MOV

#7

The seat motor is not great for a large arm. We used it in 2017 with a roughly 1 ft long arm (maybe even shorter) used to grab gears. We went through maybe 4 or 5 motors throughout season and 30+ shafts. We would twist the shafts so much that they would snap, and we used actual 8 point shafts that fit perfectly in there, maximizing contact area. It’s a long mess of dealing with fixing the problem but we designed ourselves into a spot where we couldn’t really fit any other gearbox/ motor, so we had to keep finding ways to get this to work.

#8

Where did you guys get such a shaft?

#9

We made it ourselves. We have a sponsor with a CNC mill (may have even been a 4th axis but I believe we did make some using a standard Bridgeport also) that was able to produce them for us.

#10

Thanks for all the reply. Team 1241 advise us to use the https://www.andymark.com/products/0-5-in-hex-adapter-shaft-for-bosch-motor​ . we will try it.