Which bearing parts should I use retaining compound on ?
I'm building my first AR4 MK3 and pretty comfortable with the electronic part, but lack knowledge on best practices regarding mechanical fit of bearings.
Here is the context : To assemble the J1 and J2 joints I have a slightly loose fit between the bearing parts and the housing & axis. So I bought loctite 648 to prevent the bearing races from moving or rotating within the housing. Before assembling I double checked how to assemble and notice there are screws meant to place tension on the bearings. Which I understand if the bearing are pressed but I don't know how it will be affected by using loctite.
To illustrate and ease the exchange I have drafted the attached diagram to illustrate the 4 places I intended to place loctite. 1 & 4 being between the bearing and the axis, 2 & 3 being between the bearing casing and the external housing.
So my question based on that is, should I
- glue all of the bearings and screws will still have an effect ?
- glue all of the bearings and not place the screws ?
- glue all but not n° 1 and 4 and use screws to tension but risk friction if it moves ?(given the loose fit it will probably)
... any advice on best practice ?
I recommend using Loctite 638 or a similar bearing retaining compound. When the robot was first released, the parts were manufactured with a tighter press fit. However, I found that many hobbyist and educational builders didn't have access to high-quality bearing presses and appropriate press adapters. In many cases, this led to misaligned or wedged bearing races, which could damage parts and cause unnecessary frustration.
To improve accessibility and reduce build difficulty, I adjusted the tolerances slightly. This makes the assembly process easier for more users and also helps keep machining costs low. However, it does mean there can be minor variation in fit.
Using a retaining compound will help secure the races and prevent common issues during installation. I would use compound only on the outer bearing races, and only where needed. In your diagram this would mean applying compound to the races labeled 2 and 3. Please let me know if there are any details I've missed or if you have any follow up questions.
Thank you,
Chris
Hi Chris, thanks a lot for your comments. Indeed this tolerance adjustment is very appreciated as I don't have access to a precise press, so a slightly loose part as I have is much easier for me to deal with. Great call.
I will proceed as you advised and let you know if I face other issues doing so. Also, while I have the opportunity, thanks for sharing your know-how with others and assembling/maintaining this quality kit. Can't wait to play with the final result 😃
Hi,
Feedback from a recent AR4 assembly:
I used loctite 271 when needed, meaning when the adjustment was slightly too loose.
For some bearing, using a deep freeze to make the bearing to "shrink" and water boiler for the aluminium part to expand, helped.
Anyway I also needed the help of a vice to properly install the 688Z bearing into the J5 housing
General review : the quality is excellent and thanks to provide parts with such a tolerance.
Regards
Gilles
Thanks for your comments. Now I fear reaching J5 ^^"
Jokes aside, I ended up using the Oven for J1 casings and that was sufficient. Same for J2. On the other hand I had a lot of slips on the axle (junctions 1 and 4 on the original diagram). Made some tests with a alignement marks and assembling / disassembling to confirm. So I ended up using loctite on location 1 as well, not on 4 so I can still disassemble. It is now pretty smooth and sturdy.
One concern though : The key of J2 gearbox axis does not fit at all and is skewed. It does not seem intended but the keys were already installed so I'm not sure. I attached a picture.
Is it supposed to be this way ? I tried to pinch it back to place without success as I fear to damage the axis. Is it the right way to proceed, with more force, or could it be an incorrect alternative key that I need to replace ? It has a whole in it also unlike the other motor keys.
Hi,
Hope I am not going to frighten you :-) but for J2 gearbox key, as I remember I had to use a vice, otherwise it was impossible to install. The key on your photo on the right should not be in that position..
One tip to remove it : the small hole on top the key could be use to extract it using the right diameter screw. Then use a vice to properly install it.
Hi Gilles, Thanks for confirming there is something wrong here. I guess factory defects can happen. I think the hard part will not be the vice for this one but the extraction without forcing on the gearbox ... I'll try at work with a colleague. Thanks for the tip.
EDIT : Just realized the screw is not meant as an anchor point to pull on the key but rather to screw is further until it pushes the key out. 😅 Every day a bit smarter I guess ^^
Hi Pierre,
I usually name such process " learning by mistakes". I did a lot of mistakes :-)
Anyway I should have be more precise for "the small hole on top the key could be use to extract it using the right diameter screw". Good news, you found by yourself the right way to use it.
Two more tips :
- If you do not have a vice you might use a clamp to install the key. The clamp has to be a "strong" one.
- vice or clamp: do not forget to protect the motor axle by using some kind of rubber jaw
Hope it helps.