Motor Interface

As discussed in the motor mount page, we decided to mount the motor onto a link in the mechanism due to other design constraints which made other proposed mounting locations unfeasible.

The major problem associated with mounting the motor on the link is that the direction of the output must now be turned 90 degrees in order to rotate the driven link. Our first plan was to order a set of spiral bevel gears used in an angle grinder. This particular gear is ideal for this application for a number of reasons:

the input axis would be 90 degrees offset from the output axis
spiral bevel gears transmit motion very smoothly without any harsh impact
spiral bevel gears feature very low backlash
the pinion gear and output gear could be sized in order to allow for gear reduction

Unfortunately, spiral bevel gears are typically custom made for the desired application. The only affordable gears available come as replacement parts for broken angle grinders; they are cheap soley due to the shear volume of angle grinders produced. We were unable to procure the necessary spiral bevel gears in the time frame allotted.

Instead of the spiral bevel gears we really wanted to use, we purchased an off-the-shelf right angle gearbox intended for use with a drill. The part is shown below.

This part has the desired aspects that it changes the axis of rotation by the required 90 degrees, but it has a few downfalls:

the overall length is over 6" compared to around 3" for the spiral bevel gears
this product uses cheaper bevel gears which allow for more backlash and hard impacts when the direction is reversed
the gear ratio is fixed at 1 to 1

Despite the downfalls, we were able to incorporate this product into our mechanism, although another severe issue did arise.

Unfortunately, there is no good way to interface the output of this right angle gearbox to a flat link. The output of the gear box is a hex shaft, which locks in place fairly well. The problem really exists in attaching a hex shaft to a flat link. Our original plan was to simply broach the link with a hexagonal hole. We did not feel comfortable with the strength of simply broaching the thin steel strip however, so the decision was made to reinforce the steel with a block of aluminum. We would broach the aluminum block then fasten it to the the steel with countersunk machine screws. Again, our design was thwarted when the machine shop did not have the hexagonal broach we required.

Our solution was to make the part shown below. We milled out an entire slot instead of a hexagonal hole and included a perpendicular through hole to the slot. A machine screw placed in the hole was tightened down, which slightly bent the aluminum and thus clamped the hex shaft. This solution works, but it is not ideal. We would have preferred to procure a high quality socket head cap screw and threaded a steel insert into threads in the aluminum. In this way, we would have been able to achieve more deflection on the the aluminum and thus clamped the hes shaft tighter making the joint more secure.

The next challenge we faced was interfacing the motor to the right angle drill attachment. We again suffered from lack of quality materials in the machine shop; we tried to build a coupler but were forced to use steel, which proved to be incredibly heavy. Also, the inability to broach a hexagonal hole in the part we did make killed that concept. Instead, we found an analogous part at home depot, a deep hex socket. The socket inputs a 1/4" square drive and has an output of a 1/4" hex hole; the part is shown below. We modified this part by taking down the overall length and drilling and tapping holes to include set screws. The set screws apply a force on the shaft which does not permit relative rotational velocity between the coupler and the shaft. This part works well, but is again not ideal. Unfortunately, the hex hole is only about 1/4" deep, which means that the set screw has a very hard time gripping enough material to really hold the shaft in place. The small depth of the hex hole combined with the hex hole being slightly too large (a designed feature which is great for its intended task of gripping bolt heads but not idea for this application) means that the hex shaft going into the right angle attachment wobbles. This is probably not a huge deal considering the speeds and net roation we will be using, but is a feature we are not too thrilled about.