View Full Version : pre-rotator clutch question
07-09-2005, 08:21 PM
My prerotator on my RAF has never been agressive. I used to get 140--maybe 150 out of it. Lately it has gotten worse. I was over in Indiana on a short grass strip and I had to make an extra runway pass to get the rotor up to speed. I had taken the clutch apart today and the disc pads looked really glazed.
I roughed them up with a wire wheel in a drill...and roughened up the steel plate.....but the rpm's would not wind it faster than 100 rpm.
Obviously I need new pads.....are these available at NAPA or some other automotive store?
Also...should my flat steel disc be sanded down smooth or left with a some type of hone marks for best friction?
My prerotator cable has a gentle curve to it....it has been lubed and rotated every 25 hours.
I needed those extra rpm's today.
Thanks for any suggestions.
07-09-2005, 08:30 PM
Stan I am not familiar with the RAF clutch, but most clutches and brake pad type linings do not like to be overheated, when they are they seem to become hardened, and get "hot spots" where the plate and pads become glazed, and don't grip like they should. I would leave some "machine marks" or "hone marks" on the plate, like when you have a brake rotor turned. New pads would help.
07-09-2005, 08:51 PM
Scott: Thanks for the input that reaffirmed what I was thinking. I just will see if I can find some pads somewhere.
Harry...Ken....where would I go for these pads locally? or do they have to come from RAF?
07-09-2005, 09:06 PM
Stan, the pads are standard aircraft brake pucks, either Cleveland or the other brand, Rapco I believe. The Rapcos are cheaper and appear to be the same quality. I don't know the stock number as I just took one to my local aircraft supply store and they matched them up.
The weak link is in the activating system. I altered mine to be hydraulic and get over 200 rrpm. Proof attached.
07-10-2005, 02:43 AM
When ever mine won't excead 150rrpms [glazed]I use a 4" disc grinder to lightly rough up the pressure disc, not the pucks, and I can usualy get bout 180rrpm just by don't that.
Mine's also bout 300 hours old and I reckon it'd be right for at least another 1000.
07-10-2005, 04:01 AM
Ken: I was hoping those pucks were automotive. I dont know of an aviation store nearby.
Birdy...I roughened my disc up with a dremel tool....but I will try the disc grinder.
07-10-2005, 08:33 AM
Some time back someone on the forum suggested using a 4 1/2" grinder to cut 8 grooves in the steel disc evenly spaced (45 dgrees apart) from the center. I did that and the improvement was surprising. No problem getting 150+ RRPM.
07-10-2005, 08:42 AM
Stan, trace one of the old ones on a piece of paper and e-mail or fax it to Aircraft Spruce. They'll match it up for you. Probably any of the catalogue houses will.
07-10-2005, 02:49 PM
I'm runnin' late today...had Homeland Security Patrol.
Stan, just a couple suggestions, if you will.
I don't do anything fancy with my drive disk...and I have the original one piece impregnated pad. I just remove the glaze using heavy grit emery cloth and rubbing the disk across the cloth on a flat surface.
You may have done these already, soooo...
One suggestion...Check the squeeze cable at both ends for broken strands and/or stretching.
Another ...Have someone sit in the cabin and squeeze the handle while you grab the prop and totate it back and forth...checking that the drive disk and the pulley move the same with no slipping.
Finally...pull the inner p/r cable and inspect that it is still tightly wrapped and no broken strands.
Anyway...I hope you find what's causing your problem.
07-10-2005, 03:50 PM
using a 4 1/2" grinder to cut 8 grooves in the steel disc evenly spaced (45 dgrees apart) from the center. I did that and the improvement was surprising. No problem getting 150+ RRPM. Cutting grooves in a disc to increase its grip is actually a fallacy. The grooves in production discs are there for other reasons - cooling, clearance of debris and less chance of buckleing.
The extra grip temporily noticed is from the roughness of the grooves which in turn causes extra undue wear on the pads. If the roughness ever wears smooth, then the grip will be less than origional. Maximun grip is obtained with the maximum surface area contact available.
Slipping clutches require either more surface area contact or more surface pressure.
07-10-2005, 06:52 PM
But, Tim.......too much surface area can retain too much heat. I believe that's why they went to the 3 pucks, to reduce heat build-up. When I tried the deluxe truck-clutch pad, it fried up like a marshmallow in a vat of boiling grease. They crisped right up.
07-10-2005, 07:37 PM
Thanks for the suggestions. I broke the glaze on the pucks...and put the disc on my disc sander and did a nice job of making some hone marks.
It feels like it has more friction.....I havent tried it yet as I am doing some preventive maintenance on it this week.
07-10-2005, 07:54 PM
I was thinking of drilling a bolt-hole through the whole clutch assembly and inserting a bolt to lock the clutch-face and disc-plate together. I get a pretty easy start with my hydraulic system, and SC blades and head. Then at about 500 rrpm, I'd pull the bolt out with the wire running from it to the cabin and be on my way. Wait....the wire would get all twisted around the housing. How about one of those quartz rev counters that Al and Chuck confused me with? At a certain rpm, a hammer would strike a .22 cal. shell, blowing the bolt out........through the prop. No, that won't work either. I'll think of something to positively lock the 2 surfaces momentarily together and make my fortune selling gyro parts.
07-10-2005, 09:31 PM
Stan, I don't know what system you have. But I was very impressed with setup Ron Menzie had on his Raf. He was swinging Sport Rotors as well and he had to oil them from time to time with WD 40 to allow the clutch to ease into lock up. It would spend the blades up to 125 so easy and then he would just taxie, but the motor was just idleing at 1200 rpm. I will post a picture if I can find it.
07-10-2005, 09:36 PM
Here it is.
07-10-2005, 10:46 PM
Mark, what did he have to oil? That looks like the standard RAF setup.
07-11-2005, 03:50 AM
Mark: That is my setup.
07-11-2005, 08:30 AM
He sprayed WD 40 on the pucks or friction material. I don't see how you are not able to engage your drive to a lock up status unless possibly you are engaging at too fast motor rpm initially. That set up is the same principle that an automotive clutch would use. the entire rotating disc engages the entire driven plate. Perhapes the oil is allowing the disc to slip enough at low rpm to engage smooth so it does not stall the motor at low rpm. I was very impress at how light a pressure it took to engage the rotors and how fast they came up to 125. It would have been easy to go higher but there is no need with the 5K runway and facing into the wind the rotors gained rpm rapidy and we were off the ground. WOW!
07-11-2005, 09:39 AM
There is not enough pressure on most setups between the pucks and face-plate and they let go. Automotive clutches don't use thin cable. The cable doesn't give enough grip. There are a few unexplained exceptions. I saw 200 on an RAF with the standard setup. His setup was no different than mine, but I could only get around 125. A hydraulic actuator is the answer guys. I posted an offer to get the brackets made and a source for the hydraulic stick-grip and slave, but no one seemed interested. You're all spinning your wheels and convincing yourselves of being happy with 125-150, and I posted a photo of me getting almost 240 rrpm. Once I sell the gyro and get the J-2, my interests will shift to a different gyro and this will fall by my wayside. Here's the photo once more. And yes, this photo is with the blades attached.
07-11-2005, 04:38 PM
something to positively lock the 2 surfaces momentarily together and make my fortune selling gyro parts.Ken, it is poor pad material that causes them to fry. The material should be able to handle a glowing disc.
There is such a system used in car racing where the clutch is depressed the first stage and the friction causes the "units" to synchronize in speed and then the clutch is depressed fully and the driven plate and the drive plate engage a dog clutch in the center that locks them together until the clutch is released - zero slippage. In the stock cars it works the other way (the clutch is fully depressed to bring the driven shaft up to speed and then is fully released to engage the "dog" clutch) but it can be designed to work either way.
07-12-2005, 12:26 AM
Tim, that would be great if someone could make one for a gyro that didn't cost more than the gyro.
07-12-2005, 05:36 AM
I am not an engineer but it would seem that if we continue to remove the slippage from our prerotors we are adding a sizable amount of torque to the mast tube. Something has to break. Slippage is your friend unless you redesign the mast as well. Just my own opinion
07-12-2005, 07:38 AM
I am not an engineer but it would seem that if we continue to remove the slippage from our prerotors we are adding a sizable amount of torque to the mast tube. Something has to break. Slippage is your friend unless you redesign the mast as well. Just my own opinionMark, slippage can be good but in this case it is a heat producer, inefficient and keeps the potential max RRPM from being reached.
Yes, the mast has a safe, sustainable torque limit which could be expressed as the horsepower it can withstand without damage. I suspect Chuck B., or one our other experts can do the math and tell us what that is for our standard masts. It would be helpful if an approximation of the horsepower needed to spin to say 300 rrpm, or any other rrpm, was provided. This might not be practical since I suspect that different blades will have different requirements. If we knew that the mast's practical horsepower input limit was then we could determine what the max rrpm we should limit ourselves too.
Okay, experts, any way to generate a spreadsheet where the varibles can be plugged in so pre-rotator horsepower at various rrpm for any blade make can be determined?
Since the inertia is going to be different for different makes of blades, then how engagement of the pre-rotator is handled also may be a factor in component damage. This is why users of electric pre-rotators have an interest in soft start systems. The soft start is the equivalant of slippage in other drive systems.
I see the initial torque as a source for potential damage but have a feeling that it drops off pretty quickly as the rotor starts turning. Then the only concern is whether the mast can handle the torque/horsepower required to turn a specified rrpm.
07-12-2005, 08:44 AM
With the RAF setup, 3, 2" tubes riveted between the cheek-plates, increased torque can almost twist the rotor-head around with a better prerotator. Way too much flex. I replaced them with one, 2" by 4" aluminum tube running from top to bottom between the cheek-plates. This makes it very solid and eliminates the twisting/flexing of the plates when starting to prerotate. By pumping the engagement handle, a soft start is easy. You just slip it slightly until the blades get just a little speed up.
07-12-2005, 10:08 PM
Here's a photo with the aluminum tube going the full length between the cheek-plates. Makes a world of difference as to eliminating twisting compared to the 3 tiny pieces previously. It's solid as a rock.
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