Planetary or Spur Gear?

[sqdqo]

Are planetary or spur gears better on a compact utility? I've read where planetary are more durable, I assume this give better longevity? Is the cost difference worth it.
How do bull gears differ?

 

[arlen4720]

Planetary gears are stronger because there are 3 points of contact with the gears. I think the higher cost would be worth it. I'm not sure what a bull gear setup is...I've seen the term though.

 

[Jim Timber]

Bull gear is probably a reference to having the final reduction in the transmission rather than at the axles.

Planetaries have more than 3 points of contact, they have three gears which each have roughly 3 teeth in partial mesh with the sun gear at any given time, and that's the weakest engagement of the 6 interfaces within the planetary system.

In contrast, even the helical teeth in a ring and pinion system will only have 3 teeth in contact unless you get into really large pairs or small reduction ratios.

 

[RickB]

Bull gear final drives have been successful for about 135 years of tractors. As with any design style, if engineered properly, they work well. The tractor industry has seen plenty of planetary setups that were reengineered after production began. There are many more important decisions when choosing a CUT than final drive style.

 

[Baby Grand]

In general, planetary (epicyclic) gearboxes can carry higher power density (Hp/lb) than a two spur gear train of similar weight.
But the answer to your question - which is better - really depends on the application.
One major difference is that epicyclic input and output shafts are on the same axis (co-linear).
A two spur gear train has parallel, offset axes:



The offset can be advantageous to a tractor designer if ground clearance is desired as in the final drive for the rear wheels:

 

[john_bud]

If the design or materials is "cheap" as in flimsy - under engineered, that's a bigger concern. Bean counters will shave costs untill the cost of warranty repair exceeds the savings.

 

[paulfun9]

If a choice was given per machine I would take a planetary over a spur any day.

 

[arlen4720]

Planetaries have more than 3 points of contact, they have three gears which each have roughly 3 teeth in partial mesh with the sun gear at any given time, and that's the weakest engagement of the 6 interfaces within the planetary system.

Yes sorry, I meant 3 gears are in contact.

 

[MHarryE]

I had failed planetary parts on display all over my office and at home until I retired and my wife made me get rid of most. Planetaries are nice, can provide higher gear reductions in more compact designs, but have lots of parts. There are the gears, but gears rotate on bearings, and the bearings are between the gears and supporting shafts, all which must be machined and heat treated, then ground to near perfection. The gear train is only as good as the weakest link. If the design capability is identical, the simpler design will be more reliable but usually there are additional parameters involved like space constraints. I know companies will promote their planetary design - I know, I helped write the BS because it sounds good. What really counts is whether or not it will do the job. What I often worked with required 60:1 to 110:1 reduction ratios and to get a compact design that will handle the torque dictates planetary drives. More important is the manufacturer's reputation.

 

[MinnesotaEric]

Are planetary or spur gears better on a compact utility? I've read where planetary are more durable, I assume this give better longevity? Is the cost difference worth it.
How do bull gears differ?

Insofar as tractors are concerned, bull gears and spur gears are the same thing. A spur gear consists of the pinion gear driving a larger bull gear. Technically, spur refers to the shape of the gear tooth itself where the outside tip is narrower than the tapering fatter base. The gears are tapered to make better contact and cut helically to provide a larger tooth to tooth contact for a stronger gear in any given space constraint. Spur gear sets are not necessarily limited to a single pinion and bull gear and can consist of many more gears working in unison such as in differentials.

As for what Harry said, I once got five feet off of the ground in a UH-1 that had its brand new [sic] replacement planetary gear set blow out. Apparently the company hired to recondition the transmission's planetary set didn't understand that you cannot mix new gears with used gears. As soon as the transmission was loaded and asked to take military power, ka-boom. There was a big fuss about the problem back in the late 80s or 90s.