paragliding training center
by Had Robinson
Correct belt tension will deliver the engine's full power to the propeller and help prolong the life of the belt and the redrive bearings. Go here for detailed information on belt wear issues (from Gates). A loose belt also makes an unclutched engine harder to start.
New belts loosen up quickly. Be sure to re-tension new belts after an hour or two of use.
A reliable sign of a loose belt on a unclutched engine is that it will chirp at idle. If your engine is chirping, tighten the belt ASAP.
Clutched engines, on the other hand, cannot chirp at idle because the propeller is not engaged. The chirp is caused by the sudden slipping of the engine pulley and the belt due to the variation in piston speed at low engine RPM (idle). Constant chirping (or squealing) at all speeds, however, is a likely indicator of pulley misalignment. If the belt is too tight, it will heat up and cause it to wear out more quickly and may damage the redrive bearings.
Give the belt a visual check before use to be sure it is not frayed or about to fail. A belt failure could be a disaster by allowing the engine to quickly over-rev and, possibly, self-destruct.
You can check the approximate tension on a clutched engine by twisting the belt. It should twist about 40 degrees and no more. This is handy if you do not have any tools about, but it is not accurate.
photo courtesy of Fresh Breeze
NEVER USE BELT DRESSING OF ANY KIND ON A RE-DRIVE BELT!
DO NOT PUT THREADLOCK ON THE SCREW USED TO TIGHTEN THE HOUSING ON THE CONCENTRIC SHAFT!
The instructions and steps below are for the Minari, clutched and unclutched. Other motors are similar.
Refer to the Minari engine diagram for parts identification, as needed.
If you have an unclutched engine, you can accurately check the belt tension quickly. Use a beam-type torque wrench with the correct socket to start turning the drive pulley while holding the propeller. The pulley should start to slip when the torque wrench reads at least 18 Nm (13 ft. lb). This is the maximum low value. I personally set my belt tension to about 25 Nm. 28 Nm would be a maximum value. In any case, use the value specified in your motor's manual, if given. If your belt chirps, increase the tension a few Nm's. Adjust and tighten the concentric per steps #'s1-3 and #8 below, as needed.
It is important that the pulleys be perfectly aligned. Take a straight edge (a steel ruler or the sliding ruler of a mechanic's square) and place the thin edge on the larger pulley so that it touches as a great an arc as possible on the flats of the pulley. While holding the straight edge firmly, turn the pulley so that the straight edge moves towards the small pulley. The straight edge should just kiss the flat sides of the smaller pulley. This test ensures that the pulleys are not only parallel but not offset. If the pulleys are out of plane, the entire pulley bracket (part #24) must be replaced. (It was likely bent from a prop strike to the frame or the ground.) If the pulleys are offset (top pulley moved further to the rear), a shim must be installed between the concentric and the holder so that the pulleys are not offset. Shims are available from Miniplane-USA.
On clutched engines, the drive pulley will turn freely so the above method will not work. The steps and photos below are of the Minari engine, other engines are similar. In particular, the method used to clamp the concentric so it will not turn may be different. Be sure to have the correct torque values needed for tightening the concentric on your particular engine.
1. WD-40 Locate the re-drive shaft concentric part #82. It slides into part #24, the redrive housing. There is a slit in the housing above the concentric. Spray some WD-40 into the slit so that the concentric can turn relatively easily. The slit is right above the redrive shaft clamping screw (#26). DO NOT USE OIL! If you use oil, the concentric will slip even when the housing is clamped down tight on the concentric.
Note: The engine below is unclutched but the steps are the same.
2. Loosen the redrive shaft clamping screw (#26) and the set screw below it, if there is one, so that you can easily turn the re-drive shaft concentric (#82) with the hex bit. The re-drive shaft concentric is accessible from the propeller side of the redrive in the center of the propeller mount. It must turn smoothly and easily so that the belt tension will be set correctly.
3. Loosen the concentric With the 6mm hex bit and torque wrench, turn the concentric in such a way as to loosen the belt. Depending on how the belt was installed and tightened, the concentric could have been tightened CW or CCW. Once the concentric is loose, use the wrench to turn the concentric back and forth in order to be sure it moves freely and easily. If your concentric turns very easily without any binding, back off on the torque setting 2 Nm (step #7). Some concentrics may have oil on them which must be completely removed or the concentric will slip when the engine is run under full load.
Note: It is a good idea to firmly hit the propeller attachment plate with a dead blow hammer to ensure that the concentric is fully seated into its housing. If it is not fully seated, the pulleys will not be aligned.
4. Set torque value Set the torque wrench to 30 Nm (22 ft. lb.) Note: Use of a beam-type torque wrench will give a more accurate torque setting.
5. Snug up the concentric Begin to turn the concentric in a CW manner (facing the rear of the engine) so that the belt just starts to get snug on the pulleys. Also be certain that the belt is correctly seated in both pulleys and that they are aligned with each other. Turning the concentric CW will insure that the center bolt in the concentric is not inadvertently loosened.
6. Position hex bit in concentric Be certain that the 6mm hex bit in FIRMLY and COMPLETELY inserted into the socket-head screw in the concentric. If it is not, it is easy to ruin the head of the screw so it cannot be removed. With your knee or foot, press down firmly on the rear of engine frame in order to hold it from moving when you torque the concentric screw and secure it at the same time.
7. Torque the concentric screw With one hand on the hex bit that is inserted into the clamping screw and the other hand on the torque wrench, turn the concentric until the torque wrench just reads 30 Nm or "clicks". Carefully hold the torque wrench in this position while tightening the clamping screw with the other hand. This procedure will set the belt tension to the correct value.
8. Set the concentric Finish by tightening the clamping screw to 20 Nm or the value given for your engine. If there is a set screw, also tighten it.
9. New belts Be sure to check the belt tension after the first few hours of use. It takes time for the new belt to completely seat in the pulley grooves and it will loosen after running a bit.