Square Baler Not Tying: 12 Causes and How to Fix Each One

The twine does not follow the exact path specified in the operator manual. A single wrong guide eyelet produces incorrect tension variation at the knotter entry point — the loop forms incorrectly or not at all, causing a consistent miss from the moment the new spool is threaded.

Remove all twine from both knotters. Using the threading diagram in the operator manual — not memory — re-thread both sides from spool to needle eye. Confirm each guide eyelet is correctly used and the twine does not cross over itself at any point. Run 5 test bales before returning to normal speed.

Metal guide eyelets wear a groove into their inner surface from seasons of twine running through them at speed. A grooved eyelet catches the twine on each bale cycle, creating friction that disrupts tension uniformity and can cut through the twine after enough repetitions. Burrs from field debris impact cause the same problem.

Run a finger around the inside of every guide eyelet in the twine path. Any groove, rough edge or burr felt by touch will damage twine over repeated cycles. Smooth minor roughness with fine-grit emery cloth. Replace any eyelet with a groove deeper than 0.5mm — the replacement cost is minimal and the symptom is permanent until the eyelet is changed.

A spool stored in damp conditions can have an inner core that softens and collapses under the weight of the twine above it. As the spool unwinds near the core, the collapsed core creates an irregular feed rate — the twine feeds in bursts and then goes slack, producing tension variation that the knotter mechanism cannot compensate for.

Remove the spool and check that it rotates freely in the holder with light finger pressure. If the core feels soft or deformed, replace the spool — do not attempt to continue with a damaged spool. Prevent this issue by storing twine spools in a dry, covered location away from ground contact. Never leave spools loaded on the machine between seasons.

Tension discs control how much resistance the twine meets as it feeds through the knotter. When set too loose, the twine feeds too freely — the loop that forms around the bill hook is large and slack rather than snug. The bill hook cannot grip a slack loop reliably, and it slips off before the wiper arm strips the knot.

Increase the tension disc spring pressure by one half-turn on the adjustment nut. Test with 5 bales. If the loop is still large, increase by another half-turn and retest. The target loop should be firm and close-fitting around the bill. Both tension discs must be set equally — unequal tension produces one clean knot and one loose knot per bale cycle.

Over-tightened tension discs place the twine under continuous tension throughout the bale cycle. When the knot tightens around the bale during ejection, the combined tension from the disc and the bale compression load exceeds the knot strength of the twine. This is particularly common when higher bale density settings are used without a corresponding upgrade in twine knot strength rating.

Reduce tension disc pressure by one half-turn. If the twine continues to break at the knot and the discs are already at minimum, the twine strength rating is insufficient for the density setting — upgrade to a higher-rated twine (minimum 130kg knot strength for high-density bales above 28kg). Do not reduce bale density to match weak twine; change the twine to match the density.

The bill hook tip captures the twine loop and holds it while the wiper arm strips the knot. A bill with a worn or chipped tip no longer has the geometry needed to hold the loop securely — the loop rolls off the bill rather than being held through the stripping action. A bill that is correct in sharpness but misaligned in angle presents the wrong capture surface to the incoming loop.

Remove the bill hook and inspect the tip face. Run a fingernail across the curved tip face — any flat spot, chip or rounding indicates the bill needs replacement. Check the bill hook angle against the specification in the operator manual using a protractor or the gauge tool if supplied. Replace both bills as a pair whenever one shows wear — running a new bill against a worn bill produces uneven knots even if both sides appeared to tie during a quick test run.

The knife blade must cut the twine cleanly after the knot is stripped. A dull knife does not cut — it pulls and saws the twine over several millimetres before it separates. This pulling action applies a sideways force to the finished knot while it is still on the bill, rotating the knot out of position before stripping is complete. The result is a malformed or loose knot that does not hold under bale compression.

Replace both knife blades at the start of every season as routine maintenance — the cost is low and a sharp blade is the starting point for reliable knotting. Mid-season replacement is needed if ragged tails appear before the season end. After replacement, verify the blade-to-twine-guide clearance is within specification — a blade positioned too far from the guide cuts at the wrong point in the cycle and produces erratic results even when sharp.

The needle must travel a precise arc to present the twine at the exact position where the bill hook can capture it. A bent needle arm — caused by a blockage impact, a stone in the chamber or a plunger strike — changes the tip position at the capture point by just a few millimetres. This small deviation is enough to cause the bill to miss the presented twine on every cycle.

Cycle the machine slowly by hand (PTO disconnected) and watch the needle tip at its furthest travel point. The tip should align exactly with the bill hook face position per the specification diagram in the operator manual. Any lateral offset indicates a bent arm. Straightening a bent needle arm is not recommended — the metal is work-hardened and the straightened point is weaker than the original. Replace the needle arm and recheck the tip position against specification before resuming operation.

The cam follower arm rides on the cam lobe surface and translates the cam rotation into needle movement. As the follower tip wears, the effective radius at the tip changes — the follower lifts slightly later on each cam rotation than it should. This timing delay moves the needle delivery point outside the bill hook capture window, producing a miss. The delay worsens as wear progresses, which is why timing faults start as occasional misses and become consistent failures over time.

Remove the cam follower arm and measure the tip radius against the specification in the operator manual using a caliper. Wear beyond the minimum specification requires replacement. Also check the pivot bearing on the follower arm — a worn pivot bearing introduces lateral movement into the follower tip, which compounds the timing error even if the tip radius is within specification. Replace both together when either is at the wear limit.

The cam lobe peak is what drives the follower to its maximum travel position, which corresponds to the needle delivering twine at the correct depth into the bill capture zone. A polished flat section on the cam lobe peak means the follower no longer reaches its maximum travel — the needle stops short of the full delivery position and the bill misses the twine. Cam wear at the peak is accelerated by inadequate lubrication of the cam surface.

With the PTO completely disengaged and the machine stationary, rotate the main knotter shaft slowly by hand and watch the follower travel. On a good cam, the follower lifts smoothly to a distinct peak then returns smoothly. A flat spot produces a hesitation — the follower pauses or barely rises at the peak position. Measure the cam lobe height at the peak against the operator manual specification. A worn cam requires replacement — the cam is a precision-machined part and should not be welded up or shimmed as a field repair.

The knotter timing is calibrated at 540rpm PTO speed. Every cam position, needle travel and bill hook rotation is timed to a specific phase of that rotation speed. When the tractor drops below 540rpm under load — because the tractor is undersized for the baler or the windrow is particularly dense — every timing relationship in the mechanism shifts. The needle arrives at a different phase of the bill rotation than designed, and the capture fails.

Monitor tractor engine RPM on the instrument display throughout the baling session. If engine RPM is decreasing during bale cycles — particularly at the peak compression point — reduce forward speed until the engine maintains rated RPM without laboring. If the tractor cannot maintain rated speed at any practical forward speed, the tractor is undersized for the baler or the crop conditions. A correctly matched square baler should operate at 60–80% of tractor PTO rated output in standard conditions, leaving reserve for density peaks.

A tractor at 100% of its rated PTO output in an average windrow section has zero reserve for the brief peak compression load when the plunger reaches maximum stroke. In heavy windrow sections — merged windrows, areas of higher crop yield, or sections where the previous rake pass created a thicker accumulation — the peak load spikes above the tractor rated output for 0.5–1 second. This brief spike drops PTO speed enough to shift knotter timing and produce a miss, even on a tractor that is nominally adequate for the baler model.

Reduce forward speed when approaching visibly heavier windrow sections — the higher material volume per plunger cycle creates the load spike. Alternatively, improve windrow consistency through raking adjustments that produce a more uniform windrow across the field. The long-term solution for operations where merged windrows are routine is to upgrade to a tractor with 10–15HP more PTO output to provide genuine reserve above the rated minimum for the baler model.