by Alex Varv
Reduction pulleys are a very essential element of a paramotor.
They transmit the power output of the engine to the propeller.
If not too stressed and correctly maintained, the reduction pulley bearings last a long time. However, due to other factors like overheating, prop strikes or poor quality bearings, the reduction pulleys may need to be serviced sooner.
One important factor, as mentioned, is the bearing quality. The bearings used on the Cors-Air engine reduction for example are a type C3 which have a built in play to resist higher RPM and temperatures. These bearings are relatively not very expensive (about $ 22 each).
There are also very high quality (aviation rated) bearings and are commonly known as high precision bearings. They are very expensive and during my research I found them at $300 each!
In this article, I will present the way I change the bearings on a Cors-Air big reduction pulley.
Other brands of paramotors have very similar principle and what I am going to describe can be applied as well.
The regular maintenance of a pulley consists of checking the belt tension once in a while and after 20-30 hours, removing the belt, cleaning the grooves of the pulleys, checking the bearing noise and correct tensioning of the belt.
Note: during the inspection, if the pulley makes ANY noise while being turned by hand, even if no play is felt, this qualifies it for replacement.
While changing the bearings, they need to be pressed in, sometimes with significant force. Any knocking on the bearings must be avoided at any cost. My method will ensure that the bearings are easily pressed in and there is no need to knock on them with a hammer in order to make them fit on the shaft, especially when inserting them.
A good quality type C3 bearing (if the reduction is maintained properly
will last 60-70 hours or more.
Next, I will present step by step the bearing removal and installation in pictures with captions to make the whole process as clear as possible.
First, loosen all the bolts (or better, completely remove them) from the pulley and reduction flange if present.
Next, turning the eccentric shaft or other device that tensions the belt,achieve the biggest slack possible.
Remove the belt and remove the pulley with the shaft.
Follow the steps in the pictures presented below:
Before we go any further, we should stress that if the new bearing is too loose on the shaft, it may turn in flight and destroy the shaft. To prevent it, depending how loose it is, I use Strong or Very Strong retaining compound as shown above. The amount must not be excessive because the compound may get on the bearing seal.
Also, since I do not own a powerful press, I insert the bearing on the shaft and try to center it by hand. Next, I use a clamp as shown in the next picture and I press the bearing as far as possible to ensure it is straight. Please note that the pressure is applied to the INSIDE part of the bearing and NOT outside rim.
Only after this operation, I press in the new bearings.
Using a bearing puller to insert the new bearings on the shaft is dangerous especially if they fit very tight. Using excessive force on the bearing rim (with the puller jaws) it MAY damage the bearing.
I found it out the hard way when some time ago, I was changing the bearings on one of my previous machines.
Once both bearings were pressed in, I found one of them, especially the first one ( which needs to travel further into the shaft) to be knocking.
To avoid this, I made a small and very easy to work tool which I insert between the bearing and the puller jaws. It is a flange or a piece of pipe that stands only on the INNER ring of the bearing. The diameter of this flange is slightly bigger than the shaft so it will allow the shaft to go through it while the bearing sinks. Regardless of the force applied, it is not transmitted to the bearing balls but only to the inner ring thus it will not be able to damage the balls during bearing installation.
This flange was bought at an ACE Hardware store for $1.20 and cut (shortened to fit into the bearing puller)
Here is a close-up picture to see that the flange touches only the INNER ring:
If the clearance between the bearing and the shaft is not too small, a regular bearing puller can be used.
However, after completing the bearing installation and before pressing the shaft with the new bearings into the reduction pulley housing, we should check if no bearing knocks are felt. The smallest knock will tell us that the bearings should be discarded and NOT FLOWN!
They may fail in flight very soon. A reduction bearing failure in flight can have very serious consequences!
Note: The following three pictures show HOW NOT to install the new bearings on the shaft with the bearing puller alone and without the small flange showed above!!!
Again, the three pictures above show the INCORRECT way to install the bearings on the shaft.
Once the bearings are correctly installed, we secure them on the shaft using one of the two ways:
a) with a bolt and washer (this system is not used anymore but I am showing it because some pilots may have this old version)
b) with a safety clip (ring)
Now, the shaft is ready to be installed on the pulley.
Having an eccentric shaft, it is not very easy to evenly press the shaft into the pulley. If it does not go in straight, it will damage the housing and cause a play.
The best way to install the shaft is to heat up the pulley to about 300 or even more degrees F with a heat gun.
Note: before inserting the shaft (by hand) it is a good idea to put a small amount of compound of Retaining compound on the bearings. I also used Strong Locktite for bolts and it worked very well.
Excessive amounts of compound should be avoided because they may get on the bearing seal!
Holding the shaft vertical and having the VERY hot pulley secured in a horizontal position, with one movement we push it all the way down into the housing. In case it gets stuck, the shaft needs to be immediately removed using the bearing puller, just as described in the first step of this article. Knocking on the shaft or bearings in order to try to make them fit into the pulley, will only AGGRAVATE things.
It will distort the housing and most likely will damage the bearings.
One reason for the shaft to get stuck is that the pulley rim was damaged when removing the safety ring. Great care must be taken during this operation. If the lip of the housing was even scratched, it needs to be rectified.
Note: if the shaft was not pushed down to the end, the safety ring groove in the housing may not be exposed thus making the ring installation impossible.
Before installing the safety ring, we must visually check if the whole groove is visible. Then the safety ring will easily snap in place.
The last step is to reinstall the big pulley on the machine
Any bolts that fasten the shaft into the reduction plate or prevent the shaft from turning must be treated with MEDIUM Locktite compound and correctly torqued after the belt tension has been established.