Most TMR mixer problems get blamed on knives, kickers, or ration ingredients. The geometry of the tub itself rarely gets mentioned. That is a mistake. The distance between the outer edge of the auger flight and the sidewall is not incidental, it is one of the more consequential measurements on the machine. Get it wrong by design or by drift, and you will see the consequences in every batch you mix.
What the Working Clearance Actually Does
The clearance band between the auger flight and the tub wall is where long-fibre material gets sheared. Baled lucerne, coarse teff, dry straw in a winter ration: all of it has to pass through that gap under pressure before it qualifies as mixed feed. Too generous a gap and the material orbits without being worked. Too tight and you are overloading the drivetrain and grinding away at the sidewall itself.
What makes this genuinely difficult is that the right clearance is not a single number. It is a ratio, and it changes with tub volume. A gap that works perfectly on a 10 m³ machine will leave long fibre unprocessed in a 25 m³ machine, because the auger diameter, the flighting pitch, and the distance from centre to wall all scale together. Treating clearance as a fixed dimension across different machine sizes is one of the more common errors baked into lower-cost mixer designs.
Decagonal tub geometry compounds this. On a ten-sided tub, the flat wall panels create intermittent pinch points as the auger rotates past each corner. That intermittent pressure is what forces fibrous material down and across rather than letting it spin freely. The corners do meaningful work, which is why triangular gussets at each corner matter: they close any gap that would otherwise let material escape the cutting zone. On a round tub, you do not get those pinch points, so the auger-to-wall spacing has to do more of the work on its own. The geometry is fundamentally different, and comparing clearance figures between the two tub shapes without accounting for that is not comparing like for like.
What Happens When the Gap Is Wrong
A gap that is too wide shows up before you measure anything. Long stems of hay or silage come out intact in the bunk. Mix times stretch. Operators run extra cycles to compensate, which burns fuel and adds wear to every moving part. In a dairy context on the highveld, where a late-winter ration is already leaning heavily on dry bales to stretch silage stocks, that unprocessed fibre is not a minor quality issue. Cows sort it out of the bunk and your nutritionist’s ration exists only on paper.
A gap that is too tight does the opposite kind of damage. Horsepower demand spikes through the load cycle. If the PTO or hydraulic system is already working near its limit, this shows up as stalling or as the tractor hunting for power on every revolution. Sidewall wear accelerates, and so does knife wear, because the cutting geometry changes when the auger is effectively pinching rather than shearing. You can chew through a set of knives in a season that should have lasted two.
There is a secondary failure mode that is easy to miss. When clearance on one side of the tub differs from the other, usually because the auger has shifted slightly on its mounting or because uneven knife wear has changed the effective flight diameter, you get asymmetric flow. Feed moves faster on the tight side and slower on the open side. The result is a batch that looks mixed in the tub but separates noticeably in the bunk. If your dry matter variation has crept up and you cannot pin it to an ingredient or a scale problem, check the clearance on both sides before you go further. You can read more about how flow patterns connect to what ends up in the bunk in this piece on what happens to ration quality after the mixer.
Capacity-Matched Augers and Why They Matter Here
The reason clearance ratios have to be calculated per machine size, rather than standardised across a range, comes down to how auger diameter and tub volume relate to each other. A larger tub needs a proportionally larger auger to maintain the same working velocity at the flight edge. If the auger diameter does not scale with the tub, the clearance gap grows in absolute terms even if the tub is built to the same proportions. Feed that needs shearing gets the chance to orbit instead.
LEO Agriculture specifies a distinct auger for each machine size in their VT and HD ranges, with calculated ratios covering auger diameter, flighting pitch, baffle heights, and sidewall-to-auger distance. The stated principle is that nothing is approximated: each dimension is calculated for that volume, tested, and fixed. That approach directly addresses the scaling problem described above, and it is the design logic behind the auger flow geometry used across the range.
Checking for Clearance Drift in the Field
Clearance does not usually fail suddenly. It drifts. A frozen-bale season in the Free State or KwaZulu-Natal puts significant impact load through the auger assembly. Knife changes alter the effective cutting radius. A bent or worn flight edge changes where the working clearance actually is versus where it was when the machine left the factory. By the time you notice a problem in mix quality, the drift has usually been happening for months.
A practical visual check takes about ten minutes. With the machine stopped and the PTO disengaged, position yourself directly above the open tub and sight down from the top of the auger to where the flight edge sits closest to the sidewall. What you are looking for is consistency around the full circumference: the gap should look uniform as you rotate the auger by hand through a full revolution. If the gap narrows noticeably at one point, or if it changes from one side of the tub to the other, you have found your problem.
On a decagonal tub, check the clearance at the flat panel midpoints and at the gusset corners separately. The gap at the corner gusset will naturally be smaller because the gusset projects inward slightly. What you are checking is that the gusset-to-flight gap is consistent around all ten corners, and that none of them show contact marks or fresh scoring on the flight edge.
After any knife change, run this check before returning to full production. Knife replacement changes the geometry at the cutting edge, and a new knife set installed slightly differently from the last one can alter where the effective flight edge sits relative to the wall. This is especially worth checking if you have moved from standard knives to the outer-section-only replaceable format, where the base plate stays in place and only the worn edge is swapped out. The base plate position is fixed, but if it has shifted over time, the new outer edge will not restore the original geometry.
Check the auger mounting bolts while you are in there. Impact loads from frozen bales loosen fasteners gradually. A slightly loose auger shaft will show variable clearance as you rotate it by hand, tight at one point in the revolution and open at another. That is a cheap fix caught early and an expensive repair if it runs for a full season unnoticed.
If your operation spans multiple machine sizes, keep a simple record of the clearance measurement at each check for each unit. Pattern recognition across several checks is far more useful than a single snapshot reading. A gap that is within tolerance today but has moved 8 mm (about 5/16 in) from where it was six months ago is telling you something, even if it has not yet crossed into the failure range. The trend matters as much as the current number.
For operators evaluating a new mixer purchase or working out which size makes sense for their herd, the underlying sizing logic is worth understanding before the clearance question even comes up. The geometry has to be right for the volume before any of this matters, and getting that starting point correct is covered in detail in this guide on how to size a vertical TMR mixer.
