This week I started work on the Bosch testing project previously discussed in Overview and Tools & Measurement posts. There will be a post on the testing apparatus I’m building, but later on.
Above is the testing apparatus in what I like to call the pile of parts stage.
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I built most of the frame last night, and started working this morning on a horizontal sled. The sled consists of supports, precision round rods, and pillow blocks with built-in ball bearing modules.
Early on I decided on 1.25″ linear railing for two reasons – the pillow blocks have 2″ spacing which will help them easily mate with 80/20 components, and because the added rigidity allows the rails to serve as structural components.
I went with Thomson closed pillow blocks, more specifically their Super Smart Ball Bushing Bearings, SSUPB20. I have not worked with linear motion components a whole lot, but Thomson has a great reputation and I figured there wouldn’t be any snags.
These bearings are said to represent a major advancement in linear bearing technology worldwide. They are also supposed to have the lowest coefficients of friction in the industry.
I cleaned the rails and mounted the blocks. They were hard to move and dragged a lot. I then lubed the blocks. No improvement. I called Thomson and spoke with their technical support. The blocks are supposed to move smoothly.
How bad were the Thomson bearings? They couldn’t slide down vertical rails under their own weight! That’s pretty bad. Horizontally, with both bearing blocks coupled together, it took over 15 pounds of weight to get them to move.
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I then spent the entire morning trying to figure out what I was doing wrong. The blocks move clunkily on their own, so it’s not as if external attachments were causing them to bind. Surely something I did was causing these issues.
Ball bearings sometimes move easier under load. So I applied a load on top of each of the blocks. Did they move smoothly? Nope.
Finally, I tried two NB pillow blocks (TWA20UU) I ordered for another part of the project, and they work beautifully well. The NB bearings are a little different in design, but both Thomson and NB bearings are designed for up to 1° of misalignment, and both have seals on both ends to keep crud out and lubricant in.
The NB bearings cannot support as heavy a load as the Thomson bearings, which probably means the Thomsons have smaller and tighter fitting bearings that create greater drag. Still, the level of friction and drag I was seeing just doesn’t seem right.
In case it’s of interest, the Thomson bearings are made in Mexico and arrived looking like they were pulled off installed equipment. They weren’t in bad shape, but they looked a little rough. The NB bearings are made in Japan and arrived with not even a scratch mark anywhere.
The rails I was using were Thomson-made as well, so it’s not as if I used off-brand rails with different tolerances or diameters.
Right now, I’m thinking that the Thomson bearings are just too heavy-duty for my application, or maybe there was something wrong with them – stranger things have happened. The NBs can support 1230 lbf of dynamic load and 1530 lbf of static load, which is a lot of force. If the NB can be easily and smoothly moved by hand, then surely the Thomson with its 2350 lbf dynamic load rating could too?
I considered that maybe the difference is how the NB bearing is a ball bearing product and the Thomson bearing is a ball bearing bushing product, but ball bushing just seems to be Thomson’s terminology for all of their bearings.
Either way, I am now at least a little bit wiser, although I still don’t quite understand why the Thomson bearings were just so unsuitable.
I suppose I could have contacted Thomson to see if the Super Smart ball bushing pillow blocks would be suitable for my application, but nothing in their design notes or spec sheets suggest they wouldn’t work with low loads. I thought I was spot-on with my initial product selection.
Now that I know the NB bearings work a lot better than the Thomsons, which could also just be defective, I have 4 more NM pillow blocks incoming from McMaster and the 4 Thomsons are on the way back to Zoro.
mike
Thats disappointing.
We use Thompson blocks all the time on our machines we build at work. But we support 100’s – 1,000’s of lbs on ours so that may be why we dont have much issues.
Stuart
I just coupled the NB blocks together and there’s a little bit of slop/misalignment leeway. The Thomson blocks were much tighter. But, I originally planned on a 4-block sled, so the self-alignment slop should disappear once everything is assembled and tightened down.
Chad
sounds like a complicated setup. I would have went with mounting the tool on a long radius rotating swing arm cutting downward. Add or remove weight at the end so that downward cutting pressure on the blade tip is equal across the tool lines. Have the arm swing parallel to the blades oscillation so that the radius of movement does not affect the cut and let them saw away at whatever you want cut…. with that item being mounted sideways.
When mounting the tool I would use a padded gel grip to simulate the tool being held by flesh. This will help damp out variations in imparted vibration by each tool type which would throw off the test if the tool was mounted rigidly to a non organic device.
Chad
Make the arm a two legged A frame swing arm to keep the end stable. Parts needed are a couple bearings at the swivel point and a couple long beams to make the A frame. Easy…peasy and cheap.
Stuart
I considered a vertical setup, but it has its downsides. Counterbalancing each tool is doable, but is an extra step I would like to avoid. For a vertical arrangement it will be a little harder to control the feeding rate.
Between designs I did carry over the idea of a weight-based system to control feed rate/pressure. It won’t be as controlled as with a powered drive system, but there are time and resource constraints. Plus, with the short travel distances we’re talking about, the feed pressure should be relatively constant once a cut begins.
My intent is to constrain lateral movement, such that a small portion of the blade does the cutting, as this allows the tool and blade to do the work.
I didn’t think to use padded gel, but I do have pipe straps, foam sheets, foam tubing, and hard neoprene tubing. The primary purpose will be to create a secure fit so as to immobilize the tools, and the secondary purpose is to dampen vibrations as mentioned. Some movement will be allowed, but not very much.
Chad
The vibration issue I can think of is that if you too rigidly mount a tool that has poor vibration isolation then it will transmit that power back to the blade and give it more cutting power. This would not be a real world result though and would reward the tool the vibrates excessively. A better counterbalanced machine will already transfer power regardless of the rigidity of the mount. I’d aim for a mount that simulates a palm grip.
Stuart
If vibration does contribute to tool power, then it would do the same for each blade. The only potential for bias would be in comparing performance of each tool, but that’s not the official objective of the testing project.
John Sullivan
In the event that you get feedback from Thompson regarding the problem with these pillow blocks, I hope you will make a follow-up post.
Stuart
I didn’t follow through. There’s more than a reasonable chance they were just too heavy-duty for my application, so I didn’t look into the matter further.
jeff
for something that looks to be $150+ id contact them again.
ive used their products before and they have required next to no force to move the bearing with no load.
i think you have some faulty bearings.
Stuart
I already returned them to the supplier.
jeff
im glad to hear that 😛
Dave Wittmann
I have noticed a lot of quality control issues with a variety of products lately.
We bought vanity for a job. When we started unpacking it at the customers home, it was falling apart. They had only installed about 25% of the screws. On the same job, we got not one, not two, but three faulty GFCI outlets. All different brands.
What happened to taking pride in your work?
Dave