Articles of Interest
Belt-Drive adjustment written by Keith Pickersgill
This is a (not so brief) synopsis of belt adjustment problems and solutions.
Most belt-drives have an offset cam on the large pulley (prop-shaft) that can
be turned with an allen Key or a spanner. This changes the spacing between the
crankshaft and the propshaft to adjust the tension on the belt.
If the belt is too loose, it slips, which quickly polishes the pulleys to a
very slippery surface - making further effort useless until the surface is
repaired by spraying with aluminium-oxide to create a new friction surface.
As soon as you realise your belt is slipping, come off the power IMMEDIATELY
and carry out an emergency landing, as any further flying will polish the
pulleys as well as burnish the belt to a hard, slippery surface (requiring a
Once the pulleys and belt loose their friction surfaces, then you are forced to
either repair the damage, or over-tighten the belt to prevent further slipping,
however the latter is not an acceptable solution.
If the belt is too tight, then the prop-shaft bearings and crankshaft bearings
carry huge radial loads and are likely to fail quite quickly.
Furthermore, an over-tight belt pulls the two pulleys together, pulling the two
shafts out of parallel, forcing the tracking of the pulleys out of parallel
which then quickly eats up the grooves of the belt. This usually leaves a
PERMANENT bend in the prop-shaft which is almost impossible to repair without
replacing it, a rather expensive exercise.
So if you are not sure how tight your belt should be, rather err on the too-
loose side instead of the over-tight side, as the damage is cheaper and easier
If your pulleys get polished by a slipping belt, then find someone with a
desktop bead-blaster (or sand-blaster) and load it with medium-grit Aluminium -
Oxide. Remove the bearing inside the large pulley, mask off the surfaces that
you do not wish to treat with ordinary masking tape, then spray the friction
surfaces (the grooves around the circumference) of both the large pulley and
the small pulley until you have a new matt-looking high-friction surface.
A well treated pulley will have much more grip than the fresh-from-factory
pulley which has just been cut on a lathe. With this new friction surface, you
can fly with a much looser belt and not experience any slippage.
Such an Aluminium-Oxide treatment of the two pulley's friction surfaces should
cost you approx R250. If you cannot find someone locally to do this, then send
the pulleys to me, I have the equipment for this.
OK, how to set the belt tension?
There are two issues:
How tight, and which way to turn the offset cam.
However, before making any belt tension adjustments, first check the tracking
of the two pulleys, that they are exactly in parallel with each other, then set
the alignment that they are exactly in line with each other. Note that these
are two separate issues. The pulleys could be out of alignment but still be
perfectly parallel. If the pulleys are not parallel, then you have some rather
expensive damage to repair. There is usually no adjustment for this.
Once you have ensured they are parallel, then set the alignment, i.e. that
the one is directly above the other. This is usually adjustable via a thread on
the large pulley (and/or propshaft), by screwing it in our out until the two
pulleys line up perfectly (assuming that the tracking is already correct).
OK, on to belt tension:
How tight? Each belt type and diameter has a specified setting. The belt
manufacturers have tables for this which you can find on the internet. One of
the best types of table, gives you a measurement of deflection that you can
push the belt with your finger, based on the free-run distance of the belt (the
length of belt not touching a pulley).
So half-way between the two pulleys, press the belt with your thumb inwards to
see how far you can deflect it. Typically, paramotors should allow approx 3mm
to 5mm deflection, but this is not a hard and fast rule. Ask your dealer,
distributor or manufacturer for the correct value here for your specific motor!
Now for the big question: Where should the cam be once the belt is properly
If you can take the cam to its maximum distance (tightest setting) and the belt
is not yet tight enough, then either the belt is already over-stretched, or it
is one size too big, or your prop-shaft has been bent towards the crankshaft
(which would throw out the parallel and MUST be repaired).
Ideally, the belt should be EXTREMELY difficult to get onto both pulleys with
the cam set to its closest (loosest) setting. It should really be a battle,
jumping one groove at a time as you rotate the pulleys.
OK, now that the belt is on, but WHICH WAY do you rotate the cam?
This is critical because the cam offset moves the center of the propeller to
the one side or the other, no longer in the center of the frame. This can be as
much as 25mm offset to the side!
There are many arguments about this.
Some people believe your should rotate the cam in the same direction that the
belt is driven (anti-clockwise as viewed from behind the paramotor), so that is
the locking mechanism comes loose, the engine's drive will keep the cam turned
to the tightened side.
Experiments have proven this is not true, that no matter which way the cam is
turned, if the locking mechanism comes loose, the belt will loosen and start
Are there other factors?
Yes. Some belt-drives rotate their cam on the same thread used to set the large
pulley's alignment. If this is a very course thread, then turning the cam to
tighten the belt, also screws the large pulley deeper onto the shaft, or
outwards away from the shaft, upsetting your pulley alignment.
If this is the case, then its unlikely that you achieve perfect alignment with
the cam set to its loosest setting. You need to establish which way to turn, to
improve the alignment. Its likely that the alignment will never be perfect,
however the belt-tension setting is more important.
If your cam is not on a thread, then you can turn it either way, left or right.
This shifts the center of thrust to the side on the pilot's back.
Is one side better than the other?
Yes, there is a difference. The Assymetric Blade Thrust causes the motor (and
pilot) to yaw to the right, inducing a left-hand bank on the wing, making the
wing steer to the left (in the absence of all other forces).
This Assymetric Blade Thrust can partially be used to counter torque-induced
turns, which make the wing steer to the right. However, the Assymetric Blade
Thrust is usually quite undesirable in other respects, so you can use this
offset thrust line to your advantage.
Once you have set your prop-vertical to the correct angle in flight, you can
see if there is a noticable wing still exhibits a noticable tendency to steer
to one side. Most likely it will steer to the right, so turn the Cam Clockwise,
positioning the center of thrust more to the Left-hand side of the pilot's
back, causing a slight right-yaw on the motor, inducing a left-hand bank on the
wing which will help counteract the right-hand bank caused by the torque
Let me know if anyone needs more clarity on any aspect of this.
Address comment to Keith Pickersgill at firstname.lastname@example.org