A posho mill stops in the middle of a milling day. The owner's first thought is the motor. The second is the fundi's phone number. Within an hour somebody is quoting for a rewind.
More often than not the motor is perfectly healthy. What failed is the drive: a belt slipping for weeks, a pulley that was never in line, a bearing growling since the last rains. Drive faults do not announce themselves the way an engine does. They degrade quietly, then stop the machine on the busiest morning of the season.
This guide covers the drive train on the machines Kenyan farms run: posho mills, chaff cutters, shellers, threshers and belt driven pumps. Tension, alignment, wear, bearings, grease and pulley speed. None of it needs a special tool.
Why the Drive Fails Before the Engine Does
An electric motor is a sealed, balanced, precisely made thing designed to run for thousands of hours with almost no attention. The drive that connects it to the working end is the opposite: rubber, cast iron, a few bolts and two bearings, all exposed to dust, chaff, rain and whoever adjusted it last.
That is why the drive is where farm machines actually fail. On a posho mill the motor turns the same way every day. What changes is the belt stretching, the mounting bolts working loose, the groove wearing wider and the bearings losing their grease.
The failures are also linked. A slipping belt heats the pulley. A misaligned pulley drives a side load into a bearing. A dying bearing drags on the shaft, which makes the belt slip, which makes the operator tighten it harder, which finishes the bearing. By the time the machine stops, three things are wrong and only one of them was the original fault.
The useful part is that every one of these faults is visible or audible well before it stops the machine. You need a straight edge, a grease gun, your hand and your ears.
Belt Tension: Too Loose Burns the Belt, Too Tight Kills Bearings
A V-belt does not grip the bottom of the groove. It grips the two sloping sides, and it does so because tension wedges it in. Get that tension wrong in either direction and something expensive happens.
Too loose and the belt slips. Slip is friction without motion, which means heat. The sidewalls glaze to a hard shine, stop gripping and slip more. Power stops reaching the working end, the machine chokes on a full hopper, and the belt eventually cooks and cracks. The squeal on start-up or under load is a slipping belt telling you exactly this.
Too tight is the more common mistake, because tightening is what silences the squeal. But a drum-tight belt loads the motor and machine shafts sideways with a force they were never designed to carry. The bearings run hot, the grease thins and escapes, and the bearing fails months early. On light machines the shaft itself can bend.
The field check: press the belt down at the middle of the span with firm thumb pressure. A widely used rule of thumb is about 16 mm of movement for every 1,000 mm between the pulley centres, so roughly 8 mm on a chaff cutter with a 500 mm span. Use the manufacturer's figure where you have one.
New belts stretch fastest in their first hours, so re-check after the first day and again after a week.
Alignment: Two Minutes With a Straight Edge
Tension is worthless if the pulleys are not in line. A misaligned belt is forced sideways into the groove on every revolution, so it wears on one flank, sheds rubber dust, heats up and drives a permanent side load into the bearings at both ends.
There are two kinds of error. Parallel offset means the shafts are parallel but the pulleys sit in different planes, usually because one pulley was pushed too far onto its shaft. Angular error means the shafts themselves are not parallel, usually because a motor mounting bolt is loose or the motor base is sitting on caked dirt.
Check it with a straight edge: a length of flat steel, a true timber batten, or a taut string on a long drive. Stop the machine and isolate it first. Lay the edge across the outer faces of both pulleys. If the pulleys are aligned and the same width, the edge touches at four points, two on each pulley. Three contact points and a rock means angular error. A visible gap at one pulley means offset.
If the two pulleys have different face widths, do not use the outer faces at all. Measure instead from a fixed reference to the centre of the groove on each pulley, and make the two measurements match.
Correct offset by moving a pulley along its shaft. Correct angular error at the motor mountings. Then re-check tension, because moving anything changes it.
Reading the Belt, and Reading the Groove
Belts are cheap and pulleys are not, which is why most people replace the belt and leave the pulley alone. That is backwards. A worn groove destroys a new belt in weeks.
On the belt, look for four things: a hard shiny glaze on the sides, which means it has been slipping; cracks across the inner face, which mean heat and age; frayed edges, which mean misalignment or a damaged groove; and a belt worn so narrow that it sinks in and touches the bottom of the groove. A belt riding on the floor of a groove has stopped wedging and cannot transmit power at all.
On the pulley, run a finger down the groove. It should be a clean, straight-sided V with a slightly rough cast finish. A polished mirror band on the flanks, a dished profile, or a sharp lip at the outer rim all mean the groove is worn. The quick test is to lay a new belt in: it should sit with its top edge level with or slightly above the rim, with no gap along the sides.
Two rules follow. On a multi-groove drive, replace all belts as a matched set, never one alone, because a new belt beside stretched ones carries the whole load. And never lever a belt on or off with a screwdriver, which breaks the cords inside. Belts, pulleys, bearings and grease for the machines in our machinery range are listed with prices on the price list.
Bearings Warn You by Sound and by Heat
Bearings almost never fail silently. They give warning for weeks.
Sound first. A healthy bearing hums evenly. A rumble or growl that rises with speed means the raceway is pitted. A dry squeal or chirp means it is short of lubricant. A rhythmic knock once per revolution means a broken cage or a flat on a rolling element. Hold the handle of a long screwdriver against the bearing housing and your ear against the other end and the difference is obvious.
Heat second. Housings run warm in normal work, so the absolute temperature tells you less than the change. If a housing you could rest your hand on last week is now too hot to touch for more than a second, something has changed: tension, alignment, contamination or lubricant. Compare housings on the same machine on the same day.
Then play. With the machine stopped and isolated, grip the pulley and try to rock it, then pull and push it along the shaft. Any perceptible movement means the bearing is finished. Two last tells: rust-coloured dust around a housing is fretting from a bearing running dry, and grease that comes out black and gritty means dirt is already past the seal.
Greasing: Little, Clean and Regular
More grease is not better grease. This is the most misunderstood point in farm machinery maintenance.
A rolling bearing needs its rolling elements coated, not its housing filled. As a working rule the free space in the housing wants to be roughly a third to a half full. Pack it solid and the elements churn through grease instead of rolling through it. Churning makes heat, heat breaks down the oil in the grease, and you have manufactured the exact failure you were trying to prevent. The pressure also blows the seal, which lets in precisely the dust you were keeping out.
So wipe the nipple clean before the gun touches it, because a dirty nipple injects grit straight into the bearing. Give a small pillow block two or three strokes, not twenty. If the housing has a relief, pump until a little clean grease appears and stop there.
How often depends on conditions, not on a schedule written for a clean factory. A chaff cutter in dust and a mill running eight hours a day want attention monthly through the season. Any machine washed down with a hose wants greasing immediately afterwards, because water passes seals easily.
Finally, do not mix grease types. Different thickeners can be incompatible, and the mixture can turn liquid and run out of the housing.
Pulley Sizes Set the Speed of the Machine
The motor does not set the speed of the working end. The two pulley diameters do, and the arithmetic is short: driven speed equals driver speed multiplied by driver diameter divided by driven diameter.
A common four-pole motor on Kenyan mains runs at roughly 1,440 rpm under load. Fit it with a 100 mm pulley driving a 200 mm pulley on the machine and the machine turns at 720 rpm. To make the machine faster, fit a larger pulley on the motor or a smaller one on the machine. To slow it down, do the reverse.
This matters because every machine has a speed it was built for. A hammer mill relies on tip speed to shatter grain, so run it slow and the meal comes out coarse while the motor labours. A sheller run too fast cracks and splits grain; run too slow and it leaves grain on the cob.
The trap is treating a bigger motor pulley as free output. On machines that move a lot of air, and a hammer mill moves a great deal, power demand climbs steeply with speed. A modest speed increase can push a motor past its rating, so it draws more current, runs hot and trips or burns out. If you change a pulley, watch the motor temperature and current draw over the first full working day.
Guards Stay On, and Nothing Gets Adjusted While Running
A belt drive has an in-running nip where the belt meets the pulley, and it does not care what goes into it. A sleeve, a dust coat tail, a headscarf, a bootlace or a finger is drawn in and closed on in a fraction of a second, far faster than anyone can pull back.
So the rules are short. The guard stays on. If a guard has been lost, have a mesh one fabricated; it is an hour of workshop time, and the materials sit in the workshop and industrial tools range.
Never adjust a running drive. Not tension, not a mounting bolt, not a grease nipple, and never flick a belt back onto a spinning pulley by hand. That last habit has cost people fingers on machines exactly like these.
Isolate before you touch anything. Unplug an electric machine or switch off at the isolator; on an engine pull the plug cap and wait for a complete stop. A flywheel keeps turning long after the noise stops.
Look at the pulleys themselves while you are there. A cracked cast pulley at speed becomes shrapnel, so a crack near the hub or a chunk missing from the rim takes the machine out of service. If you want a drive checked, bring the machine or a photograph of the pulleys into any of our six branches.
Key Takeaways
- Most posho mill, chaff cutter and sheller failures start in the drive, not the motor. Take the belt off and turn the machine shaft by hand before anyone quotes for a rewind.
- A V-belt grips the sides of the groove, never the bottom. Too loose burns the belt, too tight destroys the bearings.
- Align with a straight edge across both pulley faces. Four contact points is right, three and a rock is angular error, and a gap is offset.
- A worn groove kills a new belt in weeks. Inspect the pulley before you buy the belt, and replace multi-groove belts as a matched set.
- Grease little, clean and often. A packed housing runs hot and blows its seal, which is exactly how dust gets in.
- Guards on, and never adjust, grease or free a drive while it is turning.
Frequently Asked Questions
Why does my belt keep breaking or slipping off?
Almost always because of the pulley rather than the belt. A worn groove lets the belt sink and lose its grip, and a misaligned drive drags the belt sideways until it climbs out. Check alignment with a straight edge across both pulley faces, inspect the groove for a polished or dished profile, and confirm the belt is the right section for that groove. Fitting another new belt to the same fault only buys the same failure again.
How tight should a V-belt be?
Tight enough not to slip and no tighter. A widely used field rule is about 16 mm of deflection for every 1,000 mm of span when you press the middle of the belt with firm thumb pressure, so roughly 8 mm on a 500 mm span. Use the manufacturer's figure where you have one. Re-check after the first day of running, because new belts stretch quickly at first.
How do I know a bearing is failing?
By sound, heat and play. A rumble or growl that rises with speed means a pitted raceway, a dry squeal means it is short of lubricant, and a rhythmic knock once per revolution means a broken cage. A housing that has become noticeably hotter than it was last week has changed for a reason. With the machine stopped and isolated, rock the pulley by hand: any perceptible movement means the bearing is finished.
How often should I grease farm machinery bearings?
In Kenyan dust, monthly through the working season for a mill or chaff cutter in daily use, and again immediately after any wash down, because water passes seals easily. Give a small pillow block two or three strokes, not twenty, and wipe the nipple first. Over greasing makes the bearing churn grease instead of rolling in it, which raises the temperature and can push out the seal.
How do I change the speed of a posho mill or chaff cutter?
By changing pulley diameters, not the motor. Driven speed equals driver speed multiplied by the driver pulley diameter divided by the driven pulley diameter, so a 1,440 rpm motor with a 100 mm pulley turning a 200 mm machine pulley gives 720 rpm. A bigger motor pulley speeds the machine up but also raises the load, and on machines that move air the power demand climbs steeply, so watch the motor temperature and current afterwards.