Mixing bushings and bearings?

[Pseudotectonic]

I don't see the point of mixing bushing and bearings because as per airsoft physics, the differences and variables affecting the three gears do not make them considerably different from one another in terms of the type of loading and the amount of force behind them

The different surface finish on the axles themselves with different gears already makes it impossible to work out the precise newtons applying on a bearing because if it is anything but perfect it is going to have a bit of wobble, and the brand of bearing alone (and the size, and the amount and type of grease, heck even the amount of dust exposed) will determine your shock load capacity of the bearing which is what ultimately makes using bearing worth while for the speed advantage

But since every particular setup is different, even the best bearings you can find will have a risk of exploding, again because of airsoft physics and tolerances

If you have to be mathematical, the bevel gear spins the most, followed by spur and sector gear, so the bevel gear will benefit the most from the less resistance from a bearing, in theory, but if you are willing to take the risk of exploding bearings there is no reason to not use it on all three gears

So it comes down to whether your setup is so marginal that your motor and battery cannot handle the negligible amount of extra resistance from bushings that you have to sneak in one or two or three bearings to lower the resistance, and choosing how many bearings to use is just a matter of risk management, because more bearings = more points of failure

In my opinion, bushings work fine, they are more reliable, the speed difference is not worth the risks of failing bearings because they fail catastrophically in almost all cases

If you want to put in bearings, I would start with putting on the bevel gear first, then spur, then sector gear, but in reality it does not matter because again, airsoft physics and tolerances is a pure chaotic mess therefore there is no theorycrafting possible, in reality it might even come down to the finishes of your particular gearbox and whether your shimming is making one of the gears rubbing on any one of the bearings, for example, so again, no theorycrafting is possible, because it is ultimately up to you and two million other variables

 

[Sewdhull]

Yea, when you start min maxing it's clear you are in small gains territory.

Unless you can find quality bearings you are just guessing. Bushings are easy to make consistently and wear rather than fail, as are happy with loads that aren't rotational ( read side to side). Bearings, whilst efficient, have issues with loads that aren't rotational and are complicated to make. Air soft gears also tend to have variable shaft diameters which batters the bearings as the shafts wobble about.

Decent cages are essential to keep the balls in place.

Stainless bushings are essentially metal on metal, impregnated bushings like phosphor bronze self lube to a degree. Lubricating bushings is largely a waste of time unless you regularly relube them.

Bearings are very quiet and efficient ( if you lube them correctly), shielded bearings will come lubed and you shouldn't have to touch them again. Open bearings require at least the same  attention that bushings do, but they wear slowly and at low loads often don't need any lube ( lube for airsoft)

Gearboxes are very quiet with bearings and it does make it noticeable when they wear, and they will keep the gears in place better than bushings as they wear.

If you can fit 3mm deep bearings you are raising the load ability quite a bit and if you actually service your gun regularly you can keep the bearings in good condition.

For airsoft in the UK, with our low powers and soft springs a bearing should be fine... but as Psuedo says with all the variables bushings are more reliable. If you build your gun well, use decent parts with decent tolerances, service it, which might only be some correct lube in the bearings and can detect wear before the bearing cages give way you can use bearings... I do.

But I also build with bearings in the fast bits and bushings for the sector gear. If you just shoot your gun, bushings only is probably better in that case.

The one thing I would say is that bearing failure is rare, they wear for sure, but failure requires the cages to collapse, which requires either a lot of wear to allow the balls to move, or uncharacteristic loads on the bearing. But quality, installation, servicing etc etc ...

 

[Leo Greer]

In my experience with V2 gearboxes, spur gear bearings are the most likely to fail by far. Over here in America we use stronger springs, so keep that in mind, but typically if you don’t use J-caged bearings they’ll inevitably fail above about an M110 or equivalent spring.

This is a general guideline and not a rule—as other have pointed out, the individual variables are endless—I have builds running 30+ RPS and 1.5J that still use the bearings that came with the gearbox shell (Lonex). However, I’ve also failed more than one spur gear bearing at similar power.

Currently, my general wisdom is that I’ll put 3mm J-caged bearings wherever I can fit them, and if not use either FLT 2.5mm bushings or RA 2mm if I need lower profile. This seems to yield a slightly quieter box than bushings alone. Of course, this is extremely variable, depending on your shimming skill, etc, etc, but I’ve been able to achieve near noiselessness on a few builds.

Nothing wrong with a little gear noise, but the satisfaction of firing and hearing nothing but the piston hit the cylinder head is wonderful.

 

[Pseudotectonic]

In terms of mixing, it appears that ALL the manufactures (at least on Evike) would agree on bearings on bevel and bushing on spur and sector

Not sure if this means anything

https://www.evike.com/index.php?cPath=28_24_68_518

 

[Sewdhull]

I wonder if bearing and bushing placement exists because that's what people say it should be like and that's what the customer then expects, or perhaps because you can do a crap job with bushings and the gun will be ok.

A V2 or V3 gearbox has very similar gear alignment, it could be identical... I haven't checked. 

TLDR;

Radial force is a force at 90 degrees to the gear shaft, an axial force is in line with the gear shaft. The radial force on the bevel of the Bevel gear ( proportional to the tangential Force turning the gears) is a made up 1N of force to provide some illustration of the relative forces in an 18:1 gearbox, this is not torque nor what the forces actually are.

Only the bevel end of the Bevel gear bearings experience axial force, the bearings at the bevel end get an upwards radial force (1N) the other end a downwards radial force (3N). The axial force would be 2N on the bevel end.

The Middle gear has its radial forces all at one end of the gear shaft resulting in only about 25% of the force going into the bearings furthest from the gears and the force pushing up and back into the bearings at about 9.3N a the gear end and 3.1N at the other.

The Sector gear has the radial forces applied in the middle of the gear and tending to push forward into the bearing and down. It experiences the highest forces, but these are shared evenly between the 2 bearings, with about 11N each.

If you are worried bearings wont cut it, bush the Sector and the Middle bearing near the gears. If you aren't, then bearings everywhere. 

I've looked up the Newton ratings for some airsoft springs, an M120 spring was 80Newtons and someone else did some calculations for spur gears, it looks like 100N of tangential force causes 36N of radial force for those gear teeth on the web page. Looking up bearing specs it seems even the static load at 140N, rising to 460N dynamic will be fine for an 80N spring producing 28N of radial load spread between 2 bearings gives 14N for each bearing, 10% of static load and 3% of dynamic load.

Lubrication and shimming matter too, you need small <6mm shims for bearings otherwise you shim to the outer race, bypassing the spinny bit. No need to lube shims with bearings, nor gear shafts, just the bearing, regularly.

Anyway, thats probably enough of that.

Taking the Bevel gear, forces are created by the meshing of the pinion and bevel and leads to an axial force away from the pinion through the bevel onto the bearing or bushing via the shims and a radial force onto the bearing or bushing via the gear shaft again away from the pinion, the gear turns by the tangential force. This gear sees the lowest forces, axial force excepted.

The Bevel gear shaft then turns the middle gear, from its sector gear section creating no axial force, a radial force and a tangential force acting on the Middle gear. 

-The Bevel gear experiences radial forces in almost opposite directions at opposite ends of the gear, up at the bevel end and down at the sector end, and an axial force towards the bevel end of the gear.

The Middle gear has a radial force opposite to that on the spur gear part of the Bevel gear, the tangential force from the Bevel gear turning the Middle gear which creates a radial force from its interaction with the Sector gear spur and a tangential force which turns the Sector gear.

-The Middle gear experiences radial forces at about 90degrees to one another, almost at the same place on the gear shaft, up from the Bevel gear spur and back from the Sector gear spur. There are no axial forces.

The Sector gear has an opposite radial force from the Middle gear spur and a radial force from the interaction of the outer spur and the gear rack of the cylinder.

-The Sector gear experiences radial forces forwards and down, close to the centre of the Sector gear.

-The cylinder rack experiences a rearward and upwards force from the Sector gear.

View attachment 121081

https://khkgears.net/new/gear_knowledge/gear_technical_reference/gear_forces.html#:~:text=In the meshing of a,force acting on driven gear.

https://www.smbbearings.com/firebrick/ckeditor/plugins/upload/Uploads/Documents/bearingpdfs/SF693-stainless-steel-flanged-miniature-bearing-3x8x3mm.pdf

http://airsofttech.dk/Guides.cshtml?Page=SpringRating

 

[Pseudotectonic]

Only the bevel end of the Bevel gear bearings experience axial force, the bearings at the bevel end get an upwards radial force (1N) the other end a downwards radial force (3N). The axial force would be 2N on the bevel end.

How did you calculate the 3N on the other side? And the 2N axial force?

 

[Sewdhull]

For a bevel gear the axial force is around double the radial force, which I took as 1N so axial force is 2.

The calculations are done on the linked gear page.

The 3N is derived from the interaction of the Middle gear and Bevel gear 3 to 1 ratio multiplying the tangential forces and proportionately increasing the radial force.

It should be noted that gear tooth geometry affects the forces, the pressure angle in particular, but a little research indicates that the numbers on the linked page are typical.

 

[Pseudotectonic]

It seems your forces are going "one way" from the motor, what about the spring load coming back, does it apply any load on the bevel gear and does it modify the radial load directions?

For sake of my understanding do you mind drawing some arrows on this diagram?

View attachment 121089

 

[Sewdhull]

what forces do you want on there, parts are missing...is it the forces on each bearing you are after?

 

[Pseudotectonic]

Yes exactly, radial and axial forces please (some vector arrows will do)

Although I imagine in reality it is in all directions because of vibrations and airsoft physics and whatnot

 

[Sewdhull]

The forces get resolved for each gear and they originate at the motor.

Forces balance, the tangential forces that spin the gears only do so because there's more motor force than needed to compress the spring. The axial and radial forces balance at each bearing.

The forces originate at the motor pinion and build depending on the load the gears eventually connect to. The gear teeth meet at an angle which produces the radial and axial forces dependant on the gear types and gear shape.

 

[Sewdhull]

I think that's right altho i am just starting my 5th night shift. Remember these are not forces of the gears pushing against each other, they dont as they at times not in contact with each other. The forces are created when the gear teeth mesh at an angle and push at an angle creating forces in 2 or 3 directions.

View attachment 121132

The down arrow for the Middle gear i wrong, it should be up.

 

[Rogerborg]

If you want to feel that you've maximised reliability in your overpriced fragile toy that will fail in 9 other ways anyway if you actually play with it, use bushings.

If you want to feel that you've achieved that last 0.1% of performance in your overpriced fragile toy that you'll never objectively measure, use bearings.

 

[Sewdhull]

I suppose it's worth saying that bushings would end up wearing the same place making lubrication tricky and that bearings would see a load biased to one place but because the balls spread the load over an area it outer race would wear in a similar place to a bushing but the lubrication would still spread.

 

[Leo Greer]

I should mention, never trust Evike! That place is a well of horrible information. They still recommend putting silicone oil in your green gas/propane.