Again... bronze bushing vs ball and race

[frascati]

There is an older thread here that only got halfway to my question.

Elsewhere, another forum on small engines specifically, the responses never really got to the point of judging one against the other specific to small engine, 25hp and below, crankshaft journals intended for lawn, snowblower, or utility use.

I've not fully disassembled very many small engines so I was surprised to learn that a new 650 dollar B&S single cylinder 21hp OHV vertical shaft had a bronze journal on the crankshaft pto side. The print advert for this engine states....

"Heavy-duty Teflon coated bronze sleeve bearing withstands twice the belt load of standard bearings to reduce wear."

Whether compared to "standard" oilite bearings or "standard" ball and race they do not clarify but it's likely the oilites.

Some comments followed..

"Ball bearings are not particularly standard - they are premium."

"A six hundred dollar 21 horse ain't what I would call premium."


"A bronze sleeve bearing will withstand twice the belt load because it is longer (probably 1" versus 12 mm for a bearing), holding more of the shaft from movement. Being bronze, it will wear itself instead of the shaft it is holding. Think of the front auger on a 2 stage snowblower. Bronze bearings on either side (before they were replaced with plastic). Ariens still uses in their 2 stage. Hold up well with low speeds. self lubricating - once upon a time they were called 'oil-lite' or some such thing. Bronze bearings are used in MTD 2 stage on the rear wheel axle as well - outer hexagon shape to fit a stamp in the housing."

...which is not very helpful. Comparing the bronze bearings in the auger or the wheel bearings on the wheel axles or, indeed, plastic inserts? Would not each of these be improved, cost aside, with sealed two-bolt mounted ball bearings?

And....

"When I started out in this business 30-some years back most engines *I* worked on were a sleeve bearing - DU style most often, Ball bearings were a "Premium" Extra"

Again, my question is from an engineered standpoint of best performance and life expectancy since I'm asking about personal use and maintenance... not designing it for profit.

"600 bucks aint a premium when you're talking about 21 H.P. You want a premium quality 21 HP engine, you gonna be wanting to think about spending twice to three times that amount."

...once again. I tried, mostly in vain, to determine via google searches, short of actually hunting down exploded parts diagrams for a good sized sample of comparably sized engines "twice to three times that amount".

The answers seemed to suggest that oilite bushings on the output shaft were indeed superior in most respects to ball bearings in this category of engine but, counter-intuitive to that claim, that "premium" engines in this category would actually use them!

In this category of engine are high-quality name-brand ball and race bearings (disregarding cost) superior to bronze bushings on the shaft pto side?

Thanks

 

[Greenman]

I would assume a well designed ball and race bearing to be better than a sleeve bearing, but I'd also guess there are applications that would be contrary to that opinion.

 

[slag]

Ball bearings are almost always better than sleeve bearings in everything.

 

[The Boston Dangler]

Ball bearings are almost always better than sleeve bearings in everything.

that's my opinion, too. if a bearing can't hold the radial or axial load, a bigger or different type of bearing is needed. a .21 nitro engine would run like crap for about an hour before it died if bushings were used.

 

[JCH13]

Ah... the good old roller element vs plain bearing debate.

I'll outline what I know to be the pros of each one, as a practicing mechanical engineer. This excludes oil film plain bearings (like a connecting rod bearing) which are their own animal entirely.

Roller bearings

• Lowest possible drag/friction
• Functions well over a wide range of RPMs
• Self-lubricated
• Will not wear whatever they are supporting
• Generally a longer rotational service live than plain bearings.

Plain bearings

• Can carry more load for a given installation size
• Can be installed in smaller volumes/spaces than roller-element bearings
• Are generally not de-rated for non-moving contact
• Tolerant of foreign object contamination
• Excellent shock resistance
• Inexpensive
• Generally tolerant of misalignments
• Generally less expensive than roller contact bearings
• Generally much lighter than roller-contact bearings

So, to answer your question "which bearing type is best for a PTO ouput shaft?" - it depends.

If the PTO is going to see thousands upon thousands of hours of use? Is efficiency highly valued? Will the PTO output be very high RPM? If yes to any of these, then he hassle of a roller element bearing is probably worth it.

Is the PTO going to be used only occasionally, i.e. for a few hundred to a thousand hours? Is it going to be used outside or in a 'dirty' environment? Will it be exposed to large shock-loads? If so a plain bearing will work well.

I have used a wide variety of bearings in the many machines that I've designed. They each have applications that they excel in and applications that they will fail in. A PTO shaft is an interesting case because it's one of a few situations where either bearing type could be appropriate.

Grossly general statements/opinions like slag and The Boston Dangler make are exactly that: grossly general. They may be true in a particular person's limited experience, but should not be taken as fact.

 

[slag]

I've never bought anything with the intention of not wanting it to last as long as possible. I've purchased and fixed up many different items with both sleeve and ball bearings and the heavier duty items ALWAYS use ball bearings instead of sleeve bearings.

I'd never buy an item with sleeve bearings if one with ball bearings was available because, in my experience, the one with ball bearings is going to be more heavy duty all the way around and I wouldn't want to replace the item any sooner than I had to.

 

[videogames101]

Depends, high rpm applications go for ball bearings no doubt - if it's load bearing and very low rpm, it make much more sense to use a bronze bushing, think linkages

 

[JCH13]

I'm not saying that all uses of plain bearings are ideal, clearly there are many plain bearings used for cost reasons. They have their place.

In an oil-rich environment like an engine case that OP is talking about it is entirely possible that a plain bearing would operated in a full-film lubrication situation, where the output shaft never actually touches the bearing once it is up to speed.

In fact, if you want a bearing to last as long as possible it HAS to be a plain bearing of some sort. All roller contact bearings are, as the name implies, making direct mechanical contact between the ball and the race, resulting in material transfer. Plain bearings are the only type of bearing that can operate in the full-film lubrication regime, where no solid parts are touching. This can be accomplished with a pressurized oil system (like the crankshaft bearings inside the engine), oil already impregnated into the bearing material, residual oil near the bearing (splash crank bearings, like many air compressors including mine that has been running for around 20 years now), pressurized air, or even the right kind of grease will get the job done. Obviously not all of these are practical in every application, but that does not mean that plain bearings are inherently bad.

Depends, high rpm applications go for ball bearings no doubt - if it's load bearing and very low rpm, it make much more sense to use a bronze bushing, think linkages

My boss designed a special type of turbo vacuum pump, now in service on a Mars rover, and it uses plain bearings on a turbine shaft that spins at 250krpm and is lubricated with a dab of special grease. No ball bearing could meet the operational and durability specs.

It all depends on the application and the requirements.

 

[Howard]

Not even sure what the OP is asking since I've been crit by wall of text.