What's the difference in rotorheads?

[Brian Jackson]

I know this might sound like a dumb question, but what is the difference in a single-bearing vs. a double-bearing rotorhead? Yeah, I know there's one more bearing, and assume it's probably for weight bearing purposes. But I'd love to see some cross-section diagrams. In fact I'd like to see a X-section of a "standard" rotorhead for my own understanding. Can anyone link one? I'd be most grateful.

Thanks.
Brian Jackson

[Doug Riley]

Brian, I don't have a drawing for you, but the principle is very straightforward. The rotor pulls up on the spindle along a line that's usually not quite in line with the spindle's long axis. In steady forward flight, the rotor disk is angled back a couple degrees compared to the spindle. As a result, there's a bending load on the spindle and an "overturning" load on the bearing. IOW, the spindle is trying to pry the inner race out of the bearing like the lid off a paint can.

The ball bearing usually employed in gyro heads is a double-row model. Since there are two rows of balls, the bearing can withstand a certain amount of this "prying" load. Long experience with 1-place gyros tells us that the bearings are good for hundreds of hours if installed properly and kept clean, dry and lubed. The heavier loads and taller head of a 2-place machine put the bearing at or even beyond its rated ability to carry "prying" loads, however. A second bearing adds a huge amount of extra ability to bear this type of load, just as spreading out the bearings on any ordinary axle would do.

The 2-bearing heads I've seen simply have two of the usual bearings, stacked one atop the other. Nothing fancy.

[nsheryka]

i will make a cut-away view of the Solid Works rotor head i made and upload it tonight.

If you hunt arround MRC Bearing's website, http://www.mrcbearings.com/
you can find some spec sheets for just how much force some different types of bearings can safely take.

[thallett]

Doug, i don't understand how "angled back a couple degrees compared to the spindle" can have a prying FORCE when flying hands off the stick? I thought the only time that the bearing had "prying forces" was when stick pressure (force) was applied. ??

[Doug Riley]

Tom: In a non-offset gimbal head, you'd have to apply constant forward stick pressure (or use trim springs on the front end of the torque bar) to counter that skewed rotor thrust line that I mentioned.* The amount of forward pitching torque you'd be applying with the stick would be the prying load.

In an OFFSET gimbal, we deliberately offset the pitch pivot forward. That way we harness the rotor's own thrust to create that same forward pitching torque for us. We get to fly hands-off this way, but the same old prying torque is being applied to the spindle and therefore to the bearing. It's just being applied for us without any effort on our part, thanks to the offset pivot.

I'll try to make an MS Paint sketch and post it later.

BTW, I don't know of any single-bearing head that has experienced a bearing failure attributable to excess overturning load. Chuck Beaty posted the numbers on this quite awhile ago (Old Forum) and they were discouraging for heavier gyros, though.

* On the old spindle head, the pivot wasn't offset, of course. Bensen provided springs to attach to the bottom of the spindle to apply the necessary forward bias when using a joystick. With an overhead stick, the weight of the stick itself provided the forward bias, more or less. Unlike the spindle head, in the gimbal head we apply our control and stabilizing torques THROUGH the main bearing, so it has to withstand such torques.

[Jim]

If you want to see drawings of heads go to http://www.geocities.com/CapeCanaver...rotorhead.html and surf around.

Ralph Taggart also have downloadable plans on http://taggart.glg.msu.edu/gyro/kits.htm

Jim

[Doug Riley]

Here's a sketch. The point is that the rotor thrust pulls on one side of the outer race of the bearing (the thrust is applied to the bearing through the teeter towers, omitted for simplicity). This creates the "prying" action I mentioned.

We don't feel it in the stick because of the offset between the spindle and the pitch pivot.

[Udi]

Doug - if the RTV is passing through the pitch pivot axis than why do we need a trim spring? I always thought the RTV is passing behind the pitch pivot point, and the trim spring is balancing the difference.

Udi

[Doug Riley]

Udi: In most cases, that's true. A few rotorheads do have just the right offset so as not to need a trim spring (requires either less than the standard 1" offset, a very tall head or else blades that "blow back" a lot).

To keep the explanation (and the picture) simpler, I showed the "correct" offset rather than the slightly excess offset that we usually have. The point is the same either way: the thrust line passes to one side of the center of the bearing and creates an overturning moment. To refine the picture, just shove the whole spindle-bearing assembly, with the teeter bolt and thrust line, back a bit along the torque bar so the rotor thrust line falls aft of the pitch pivot. The relationship of the thrust line to the bearing doesn't change.

(Such excess offset allows us to link the airframe and the head via a spring. That's helpful if the airframe is pitch-stable but unhelpful if it's unstable.)