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Top 80 Clutch

by Had Robinson

Introduction

See paragraph "C" below for a photo of the clutch with the description and location of its parts.

The Top 80 redrive contains a simple centrifugal clutch.  As the engine speeds up, the centrifugal force is enough to overcome the force of the springs which hold the shoes (connected to the engine crankshaft) away from the bell (connected to the redrive gears and the propeller).  The faster the engine speed, the greater the force of the shoes against the bell.  Clutches of this design are immune to catastrophic failure.

Over the years, Miniplane has manufactured different clutches.  Replacement springs and links are available only for the latest clutch model.

The clutch should look like this.  This is the side away from the engine.  Note that the clutch springs can be installed on either side of the shoes.

Top 80 clutch assembly

A clutch in good condition should engage at approximately 2,500 RPM or greater.

See the SPECIFICATIONS page for the proper maintenance intervals for the clutch.

A. Clutch failure

Typically, what wears out first in the clutch are the link clips and the removable plates underneath the clips.  These are replaceable.  The links themselves are hardened tempered steel and never wear out.  The clips and plates, on the other hand, are ordinary steel and can wear out.  However, the links must be purchased as a complete matched set.  The individual parts of the link are not available.

The next part to wear out are the link holes in the inner hub of the clutch.  They will begin to undergo elongation towards the edge of the hub.  A failure of one of the link holes will not affect the function of the clutch other than increase engine vibration.  If (2) adjacent link holes in the inner hub fail, the clutch will no longer disengage at idle and it must be replaced.

The clutch shoes will not wear wear out before the hub fails.

The clutch bell has a minimum thickness.  Like the shoes, it rarely -- if ever -- will wear out.  However, some defect or damage to the clutch shoes could cause the bell to wear out quickly.  This is why the redrive should be removed and the clutch examined every 100 hours or so.

B. Clutch drag at idle

If the clutch drags at idle (< 2,200 RPM), it can have many causes, some of which are not even related to the clutch.  Clutch drag can also be due to factory manufacturing errors.

1. Misaligned redrive and how to fix

The most common cause of misalignment is a wearing away of the redrive housing where it attaches to the engine and/or the wearing away of the reductor (spacer) ring.  This allows the redrive to move off-center to the engine drive shaft causing the clutch to drag.

Unfortunately, even new engines have enough play between the reductor ring and the redrive to allow the redrive and engine shafts to be off-center enough to cause clutch drag.

At idle, the clutch shoes will be somewhat extended.  The faster the idle, the more extended they become.  When the clutch is off center with respect to the clutch bell, the partially extended shoes will contact the clutch bell and the propeller shaft will turn at idle speeds.

If this is not corrected, the link holes in clutch hub will wear out more quickly, eventually requiring a complete replacement of the clutch (the hub is not available separately).  The engine will also vibrate more due to the out-of-balance clutch shoes rotating off center.  The reductor ring must always be installed to help center the clutch on the engine.  Why there are not locating pins between the redrive and the engine case is a mystery.  Hopefully, someday Miniplane will correct this.

To fix a dragging clutch due to misalignment:

  1. Loosen the two nuts that hold the redrive nuts and then tighten them to 3 Nm.  This is done in order to allow the redrive to move slightly when struck with a dead blow hammer.  If the nuts are too loose, the redrive will not hold its position once struck with the hammer.
  2. Secure the propeller.  You can also do this adjustment with the propeller removed for better results.
  3. Start the engine and let it warm up.
  4. Increase the idle speed by turning the idle speed adjustment screw CW so that the propeller just starts to spin, then back off the idle so that the propeller just stops.  If the propeller spins no matter how low the idle speed is set, the clutch is defective and must be examined.  There must be no force against the strap or line (if there is one) holding the propeller from spinning.  I.e. the strap or line must have some slack in it.  If the engine is at 3,000 RPM or greater, you are done.  Tighten the nuts to 20 Nm.  If the propeller spins with the engine < 3,000 RPM, continue with the steps below.
  5. Remove the restraining strap   As long as you are directly next to a slowly spinning propeller there is no danger -- you can instantly reach out and grab the propeller with your hand near the center, if needed.  There is very little force on it at idle.  KEEP YOURSELF AWAY FROM THE TIPS AT ALL TIMES.  Even at idle, the tips can give you a nasty bruise.  I use my knee to hold the propeller.
  6. Align redrive   Turn the idle speed adjustment CW until the propeller starts to spin again.  Use a dead-blow hammer to hit the redrive housing in each of the (4) quadrants as shown in this video until the propeller stops spinning.  You will see that the propeller will move more when the redrive is hit (moved) in one direction or the other.  You will have to experiment to determine the best position of the redrive.  Increase the engine speed a few hundred RPM and align the drive so that the propeller stops spinning.  Continue this process until the engine is at 3,000 RPM or more.  If the propeller does not spin when the engine is at 3,000 RPM, you are done and can tighten the nuts to 20 Nm.  If it spins at a lower speed but > 2,200 RPM, the clutch springs are weak or you may not have properly aligned things.  If it spins < 2,200 RPM engine speed, the clutch is defective and must be examined (see below).

If Miniplane had installed pins between the redrive housing and the engine crankcase, the alignment process given here would not be necessary.  The pins would make absolutely sure that the engine and clutch shafts are perfectly centered.  As it is, the alignment can be off many thousandths of an inch whether the engine is new or not.

2. Clutch bell and link clearance failure

The tip of the link on the open side (where the clip is) can extend sufficiently to rub against the clutch bell and drag.  To fix this, reverse the links so that the clip side faces the engine side.  If you want to ensure that the springs are on the "right side" of the clutch, the entire clutch shoe assembly can be flipped 180º so that the link clips face the engine side.  The other side of the link has much less clearance.

How did this happen?  The clearances in the clutch are very tight.  When the clutch was engineered and/or manufactured, the tapered hole in the clutch hub was not cut deep enough.  This defect places the clutch too far to the rear of the engine where it contacts the end of the clutch bell shaft.  The result is that the clutch will drag -- a nuisance that will not cause any noticeable long term problems.  There will be axial forces transmitted to the main engine bearings but they can handle it i.e. the motor will have to be completely overhauled long before the main bearings wear out from axial forces.  This defect can also result in the problem discussed in paragraph 5 below.

defective Top 80 bell housing

3. Spring failure

The springs will weaken over time.  Remove the springs and measure the length of each between the points shown by the red arrows.  If one or both has an overall length > 31mm, it must be replaced.  Replace springs in pairs.  The spring on the left has stretched and must be replaced.  A normal spring has no gaps between the coils.  Note that the springs must be removed to measure their length and determine if they are useable or not.

Top 80 clutch springs - one good, one bad

4. Link hole failure

If the link holes in the either the clutch (less common) or the inner hub (more common) are badly worn, the physics of the clutch changes.  However, this should not cause the clutch to drag at idle but it may cause the clutch to engage at less than 2,500 RPM.

If the link holes wear completely through, the links will rest against the inside surface of the clutch shoes.  Things might eventually wear enough to jam the clutch shoes against the clutch bell.  The clutch will always be engaged from that point on.  It would take a great amount of time for this to happen and I am not sure the clutch bell would be deformed.

This inner hub was about to fail.  Notice that the links are not worn at all.  Periodic use of anti-seize compound would have prevented the hub from wearing this badly.  This clutch would continue to work for a while.  How long?  (But why find out?)

Top 80 clutch inner hub that is worn out     Top 80 clutch inner hub that is worn out

The arrows in the photo below show the engineering and/or manufacturing error which allows the nut that secures the clutch hub to the engine shaft to protrude out about 0.5mm.  This error may cause the nut to rub against the inside end of the clutch bell shaft, causing the clutch to drag at idle.

Top 80 clutch hub worn out and about to fail

5. Clutch bell shaft and engine shaft nut clearance failure

The clutch is mounted to the tapered end of the engine crankshaft and secured with a nut.  If there is not adequate clearance between the end of the crankshaft (or the nut) and the clutch bell,  the clutch will drag.  As with the problem in #3 above, it is a nuisance because the propeller will turn at idle and it is more difficult to make the low speed adjustment on the carburetor.

The photo below shows an engine with this problem.  The red arrow points to the wear caused by the nut on the end of the engine crankshaft rubbing against the inside end of the clutch bell shaft.  The clutch bell shaft and the nut are tempered steel so the wear of each is minimal.

The distance (depth) of the inside end of the clutch shaft (red arrow) to the exterior engine side of the redrive (black arrow) should be 30.5 mm.  If it is less than this, the nut on the end of the crankshaft (or the end of the crankshaft, itself) and the clutch bell shaft will touch.  This redrive had the correct clearance.  So what happened here?  It could be a number of things that would decrease the clearance between the clutch bell shaft and the nut on the end of the crankshaft:

Top 80 redrive clutch bell side

How can this be fixed?  The tapered hole of the clutch hub could be widened by a machine shop which could be expensive i.e. it could cost more than a new clutch.  Here are two other ways to fix this:

C. Clutch assembly and repair

If the clutch is noisy, will not engage, or is always engaged, a new clutch is needed.  I have attempted to repair these clutches over the years but without success.  In order to service the clutch, you will need the special puller that is available from Miniplane-USA.  It is a very short distance from the drive shaft to the bolt holes and most pullers will not work.  Do not be tempted to experiment with improper or makeshift tools at this point as you can easily ruin the redrive side of the engine crankshaft.

WARNING - DANGER:  DO NOT RUN THE ENGINE WITHOUT THE REDRIVE ATTACHED BECAUSE THE CLUTCH WILL EXPLODE! CLUTCH PARTS MOVING THROUGH THE AIR FROM AN EXPLOSION CAN KILL!!!

The parts of the clutch

Top 80 clutch with parts identified

1. Remove the redrive

Begin by removing the redrive and securing the clutch so that the nut can be removed.  A small oil filter wrench can be used or (much better) a tie-down strap can be wrapped around the clutch in the manner below which will keep the clutch from turning while removing/tightening the clutch nut.  Using this technique will prevent any damage to the clutch.

Top 80 clutch secured with a tie-down strap

2. Remove the clutch nut

Liberally apply some WD40 around the nut and let it sit for a few hours so it can penetrate the threads and shaft.  Remove the clutch nut.  With badly rusted or frozen nuts, you may have to apply heat for about 45 seconds with a propane torch.

3. Pull the clutch

Use the correct sized puller to remove the clutch from the tapered engine output shaft.  Miniplane-USA has this tool here (at the bottom of the page).  Do not even think of removing the clutch using any other method, you will destroy the engine output shaft and/or the clutch. Fit the puller and tighten it slowly.  The clutch should just pop off.

4. Disassemble the clutch

Note carefully the link orientation.  The clutch turns counterclockwise facing it when installed on the engine.  The leading end of the link must be set in the inner hub hole and NOT in the clutch shoe hole.  The open end of the clip must be installed so that it faces the trailing end of the link.

a.) Remove the clips by sliding them off by squeezing on the open end of the clip and link stud with a pair of pliers.  Remove the plates under the clips.

b.)  Place the clutch on a board with the springs facing down (the side that faces the engine being up, as here in the photo).  Using a small screwdriver or punch, drive one end of the spring out of the hole in the clutch shoe as shown here.  Carefully remove the spring after removing the links.  If the springs are stretched or damaged, they cannot be reused.

In the photo below, the view is of the back side of the clutch (the side facing the engine).

Remove the springs on a Top 80 clutch

c.) Once the clutch shoe tension is released, it is easy to remove the links.

5. Examine the clutch

Carefully examine all of the clutch parts for wear, especially the inner hub.  Replace parts (or the entire clutch) as needed.

6. Assemble the clutch

Assembly of the clutch is the reverse of step #4 above.  The inner hub will only fit on the engine shaft one way.

Use a small amount of anti-seize compound on the shafts of the links and under the plates to retard wear.  Too much of it will get into the clutch shoes and cause them to slip.  Every 50 hours or so, it is a good idea to disassemble the clutch and re-lubricate the links.

The shoes can be put on the inner hub with the machined space for the springs facing the engine or facing the redrive.

The links can be put in on either side just as long as the leading edge of the link is connected to the inner hub.

If the links are touching the bell, just reverse them so that the clips are on the engine side where there is more clearance.

The springs are easy to damage so be careful.  Hold the clutch in a vise.  Installing the first spring is relatively easy, but not the second.  Use a small screwdriver (the one on the right in the photo below) to separate the clutch halves just a little (twist the screwdriver 90 degrees to it stays in place) on the side that already has a spring.

WARNING: do not use a big screwdriver to separate the clutch halves or you risk overstretching the spring.  The screwdriver tip should be just big enough to separate the clutch shoes just enough to install the opposite spring.

Top 80 clutch spring installation

Using a small screwdriver, stretch the second spring.  Use the other hand to help.  Stretch the spring just enough to catch the tip of the spring in the hole.  With a screwdriver and hammer gently tap the spring completely into the hole.

The clutch shoes are different on each side.  One side has a machined out space for the spring.  Miniplane has noted that it does not matter which side of the clutch shoe the spring is installed.  The advantage of using the side without the machined out space is that it is much easier to install the spring.

Below is a new clutch with the springs installed on the side without the machined spaces.

Top 80 clutch

The clutch below had to have the links reversed so that they would not rub against the bell housing, causing the propeller to turn at idle.  Note also that the machined openings for the springs are on the back side, including the springs.

Top 80 clutch with properly installed srpings

7. Install the clutch

Install the clutch on the engine shaft and tighten the center nut to 38-40 Nm.  The use of threadlock on this nut is not necessary.

8. Install redrive with the reductor ring

Place the reductor on the engine case, the redrive, the mounting washers, and then hand-tighten the nuts.  Go to step B.1 above to align the redrive.

D. Parts source

Miniplane-USA has replacement links, springs, and the entire clutch.  Parts are not available for the older style clutches.  If there are problems, pilots will have to purchase the complete replacement clutch, which is the best option.  Thankfully, a brand new clutch is not costly.  Note that if the clutch is replaced, the new clutch may also drag if it is not aligned properly.

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