Big Blue Saw Blog
- Parent Category: Big Blue Saw Blog
- Created: Friday, 16 October 2015 19:07
- Written by Simon Arthur
A fellow robot builder once told me that using ball bearings in a waterjet cut base was "advanced waterjet mastery," which I thought was a little odd. To get ball bearings to work in a waterjet cut part isn't hard: just make a hole big enough to fit the outer race. It turns out, though, that there is a lot more subtlety to making this work the right way.
You see, without a precise hole to fit in, ball bearings tend to wear out early. Choosing a fit is complicated, and depends on the type of load the bearings will see, among other factors. (See SBP's article on the subject and the MITCalc website), but typically what is known as an H7 fit is used. This would mean that, for example, that a the hole to hold a ball bearing with an outer diameter of 0.5 inches would need to be between +0 inches/mm and +0.001 inches (0.018 mm) oversized. This type of tolerance is impossible to achieve with the waterjet or even ordinary twist drills. You'd need a very accurate milling machine, lathe, or possibly a precision reamer in a drill press.
In practice, however, that kind of precision is often not necessary. For light loads, infrequent use, or short lived prototypes, you can get away with using a low-taper waterjet cut hole that's 0.01 inches oversized. Additionally, to fill in any gaps and hold the bearing more solidly, you can use a retaining compound like Locktite 638 or Devcon 68050.
I built the gearbox shown below for my fighting robot Jaws. The bearings for the larger shaft have a 3/4 inch inner diameter and a 1 5/8 inch outer diameter. The smaller shafts, at 1/2 inch inner diameter, ride on ball bearings that have a 1 1/8 inch outer diameter.
I used the more primitive technique for this: rather than going for an exact H7 fit, I oversized the hole and fit the ball bearing in place, holding it tight with retaining compound. You can see the robot in action in the video below. The ball bearing seems to run quite smoothly in this case, even though it has been placed under some brief, intense use. I don't know if I'd trust it to work under a heavy load for a year of continuous operation, though.
Let's take a look at how the waterjet cut pieces work together. The ball bearings in this case are 3/8 inch thick. Since the bearings are thicker than the main 1/4 inch walls of the gearbox they are held in place using 1/8 inch thick plates. At Big Blue Saw, we call this type of construction the stacking technique. I tapped 3 holes into the 1/4 inch wall for the screws to hold the stacked plates in place.
Here's a closeup of one of the the 1/2 inch shafts with the shaft collar removed. If you look closely, you can see there are two 1/8 inch layers stacked on top of the 1/4 inch wall: the inner layer supports the bearing much like the 1/4 inch wall. The outer capture plate layer keeps the bearing from sliding out axially. It has just enough clearance to allow the ball bearing inner race to move freely.
Here's the assembly with the outer capture plate removed.
If you're less worried about the efficiency of the rotating shaft and don't want to bother with getting a precise fit for a ball bearing, a bronze bushing is a good alternative. Bushings are also compact, light weight, and less expensive than ball bearings.