Model Engine Maker

Help! => Specific Engine Help => Topic started by: Trevorc on January 15, 2019, 07:11:31 PM

Title: Fitting small diameter ball bearing races.
Post by: Trevorc on January 15, 2019, 07:11:31 PM
In building my previous engines i have been able to use machined bronze bushes. Now I am building the Y model Anzani I have to fit several ball bearing races with diameter below 1 inch.
I would like some advice on how to fit these in a model engine.
Do I stick with the “ correct” way by using an interferrence fit inside the housing and onto the shaft or do i go down the “ loctite “ route and effectively glue the bearings in place.
My concern is that i may be faced with having to take the engine apart several times, more difficult if interferrence fit.
What do other do?
Thanks in advance for any advice.
Title: Re: Fitting small diameter ball bearing races.
Post by: steamer on January 15, 2019, 07:36:11 PM
For small bearings I try to stick with reamer sizes.   If the bore is blind, ill bore it to size.    A push fit is what im after.
Title: Re: Fitting small diameter ball bearing races.
Post by: Tennessee Whiskey on January 15, 2019, 08:33:13 PM
I’d say that Locktite has spent billions on being able to safely call it “bearing and sleeve retainer” . If you know you are going to have several “fittings” , then I wouldn’t hesitate to machine where you can assemble and disassemble, and then rely on the product to do it’s job. I mean, most of their business is serious retention values in comparison to what we need. Just follow their guidelines. My 2 pesos worth.


Title: Re: Fitting small diameter ball bearing races.
Post by: Ramon on January 15, 2019, 08:53:27 PM

Having built several engines now I would firmly recommend you adopt the interference fit as opposed to using Loctite.

The main reason for that you have answered yourself in that to remove and replace the bearings will be easily accomplished with a degree of heat to expand the housing slightly but no where near the heat that will be required to break the bond of Loctite.

If good measuring kit is not available I find that boring the housing to a plug gauge carefully turned and polished to just under the bearing size (0.003-5mm) until it just fits will give a good indication of fit to the bearing.

Loctite Bearing fit is ideal for bearings that will sit in situ for long periods - and yes it doe ease tolerance values but it's a rather semi permanent solution and especially so in a small engine.

Hope that helps - Tug
Title: Re: Fitting small diameter ball bearing races.
Post by: Mcgyver on January 15, 2019, 09:29:34 PM

I would like some advice on how to fit these in a model engine.
Do I stick with the “ correct” way by using an interferrence fit inside the housing and onto the shaft or do i go down the “ loctite “ route and effectively glue the bearings in place.

Generally you don't want to have a press on the shaft and housing.  The starting point would be the basics of rolling element bearing installations which are covered in most bearings makers catalogues, although I grant they must employ scores of highly intelligent folks focused on making said catalogues as unreadable as possible.  However it will tell you the fit required.

Basically the usual set up is the bearing is pressed to the moving part (most often the shaft) and a tight slip fit in the stationary part (tolerance you get from the maker).  Think of an electric motor, you can manually pull the bell ends off but not the bearings off the shaft.   You can also can only press on the race that is in interference, i.e. never apply force through the rolling elements.   If the press fit being on the shaft, you can only press on the inner race to install (if installing via pressing vs shrink)

So.....even if installed the proper way, well machined fits to tolerance, the bearing is not removable from 1/2 of the assembly without damaging them.  Its extremely difficult to pull off a bearing pressed on a shaft off without putting the force through the rolling elements.

Just saying that you should understand where the interference (or loctite) joint is is suppose to be and where the slip is suppose to be, and you'll have a heck of time removing the interference fit without hurting the bearing.   The interference fit also prevents any sort of eccentricity and is imo the way to go

Machining those interference and slip fits within tolerance is a  great challenge, at least in my case, probably the most demanding work done in the home shop.  For precision bearing sets you have to hold tenths, and the smaller the bearing the smaller the tolerances.  That challenge is probably why people go with loctite.  Get a tenths indicator mounted on the cross slide and it helps to hold the tongue just so.
Title: Re: Fitting small diameter ball bearing races.
Post by: steamer on January 16, 2019, 12:33:54 AM
Small bearings used in RC cars are pretty damn rugged.  You would be amazed at the level of abuse they take in a filthy environment, subject to no maintenance, and indifferent handling.

Keep in mind that My son's truck is doing 45 Mph around the sweeper, and then going 15 feet in the air on the larger jumps in mud and what have you for many 10's of laps without issue...the wheels are only 3.5" in diameter....the motor bearings are hybrid with ceramic balls ( 5mm bore)  and are running in excess of 35000 rpm at nearly 200F for the whole weekend all the while being bead blasted with dust....

Just Sayin.....

Our little engines are pretty easy duty in comparison      All of these bearings are "slip fit" sized,  Nothing is a press fit, because of the need to service the vehicles.    The hybrid bearing has a seal on one side and a shield on the other to reduce parasitic drag at the motor.    The 36mm diameter motor is 60 mm long and is putting out over 1 HP nearly continuously for 8 minutes or more.

Title: Re: Fitting small diameter ball bearing races.
Post by: Neil-Lickfold on January 16, 2019, 06:38:08 AM
Being as the engine case wont be running all that hot,  making the housing 0.02mm on diameter smaller than the bearing outside diameter to hold it in place will work very well. If you use C3 clearance bearings, then this is not a problem. It really comes down to how much running you envisage this engine to be doing. If it is just an exercise to make a radial engine and all it does is the occasional bench run, then it wont make much difference if it is a light fit and is loctite in place. If you intend to do a lot of running with it, then the interference shrink fit is best for the bearings in the case.
For the shaft fit, a light fit, to just sliding together, is about 0.005mm in diameter smaller shaft than  the nominal bearing id. SO like  a 12X28X8 (6001) for example,
the shaft would be Ø11.995mm diameter for the area where the bearing fits. Ø12.00mm will be a definite press fit  . The case would be Ø27.98 to Ø27.975mm for a shrink fit. If the case was heated to 100c to 120c will allow it to assemble and disassemble.
Over time if  the bearing is fitted loose, especially the crank bearings, then the bearing seat in the case will slowly wear and flog out.
Another trend I have seen is where the case is bored significantly oversized, so like 4mm to 5mm diameter bigger than the final fitted bearing. That ring is a high shrink press fit, so will be 0.06mm to 0.08mm diameter press fit. Then the case is rebored to be a light fit for the rear bearing, being in the above example Ø27.99 to Ø27.995 . They do this to reduce the case from hammering out around the rear bearing. Especially in a high loaded engine like an F2C diesel team race engine.
So these sizes can be difficult to achieve and measure. Not everyone has access to a sunnen hone gauge to measure cases with.  Boring it close and then using Al Oxide paper and piece of wood, you can polish the diameter to final size. To  practice measuring a bore with a telescopic gauge, get some ball bearings with like a a 20mm id bearing (6004) and keep practising the measurement until you get a number like 20.00 to just a little but under 20.00 so like 19.995 ish. Once you learn this feel, it will give a lot of confidence to measure most things with a good telescopic gauge that others will say is not possible. Mitutoyo and Moore and Wright are good telescopic gauges. Bad ones are sticky to slide when lightly nipped, and will have a jerky feel as the gauge goes through the bore centre line.
Hope this is of some help.
Title: Re: Fitting small diameter ball bearing races.
Post by: Ramon on January 16, 2019, 08:25:59 AM
Hi Neil,

I use the basic tele-gauges to size the housings and as you say taking several measurements to be satisfied.  A quickly made plug gauge confirms it. Personally I prefer the double sliding type to the Moore and Wright type which I have had but would agree with you that they don't slide as well. I cured this by stoning and polishing the surfaces where the locking screw locates - a big improvement.

Given the type of engine that Trevorc is making - ie not likely to be used in a flying situation then I still would not go with the Loctite due to disassembly mentioned however would totally agree if being used to power a model then a sound fit as you describe is required - as someone who prefers to use Loctite Retainer in other situations eg building a crankshaft etc I still would shy away from it in this particular situation for the reasons given.

As I'm sure you would concur the difference between a good fit and a slack one is very small and smear of bearing fit can save the day however though the issue of release is still there.  On the other hand being too tight is just as easy - I find it's much easier to release a bearing from the case with mild heat than it is to get it off a tight shaft (without stressing the bearing or marking the shaft).

With the lack of accurate measuring kit this is always a hit and miss affair for me. I have found, like you, that the housings are easily tweaked with wet and dry on a dowel and the shaft dealt with by lapping or, as I prefer, stoning to the fit required - simple if tad time consuming.

Hope that helps a little more

Regards - Tug
Title: Re: Fitting small diameter ball bearing races.
Post by: Neil-Lickfold on January 16, 2019, 10:39:12 AM
A tip that might be useful when turning precision diameters is to get to know how your tools work with the particular material you are cutting. So like in boring the bearing diameter , it is a good idea to make practice finish cuts before you get there. Some boring bars and inserts can take very small cuts consistently, while others can't take a very small cut. So establish the depth of cut and surface finish that it can get very good results. Then figure out a way to make 3 finish cuts that are in the range that the tool can make consistently. You take the 3rd finish cut and measure. The remainder should be exactly 2/3 of what you had allowed for. The second cut will be exactly 1/2 of the last finish pass. Measure and it should be the same cut again to get the size you want. You can do a similar approach to precision turning on a shaft or any external part as well. With boring bars, you always get deflection, and that varies to how far the bar is out from it's support, and also varies due to the hardness of the material being cut, and it's difficulty of machining. Some materials just don't do well with small cuts. A small cut is anything less than 0.05mm on diameter. Typically a finish cut is 1/4 of the radius of the tool per side. So a R0.4 insert would be a finish cut of 0.1mm per side or 0.2mm in diameter cut. Of course the sharpness of the tool matters and the rigidity of the set up etc etc. Sometimes a finish cut has to be 0.05mm in diameter or smaller due to the nature of what is being done. I like using R0.2 insert tools for finishing, and usually make my finish cuts in 0.1mm diameter at a time. So for a bearing bore, will ruf to about 0.4mm smaller than the finished size. Then cut and get to 0.3mm smaller. Do the calculation based on the remainder as described above.
This approach is worth while for 1 off parts. Mass produced parts are not done this way.