There is a particular kind of frustration that comes from watching one end of a 60-metre bunk run flush while the far end sits half-empty. You backed off the door, you sped up a little, you guessed. The cows at one end pushed each other around for feed all morning. It is a fixable problem, but fixing it properly means understanding what the discharge door on a vertical mixer is actually doing, and why tractor speed and auger speed have to move in a coordinated relationship with it.
What the Discharge Door Geometry Does to Flow Rate
On a vertical mixer, the discharge door is not just a gate. It is the primary flow control device. Opening height determines the cross-sectional area through which blended ration can exit, and that area changes the flow rate in a roughly proportional way. Open the door a quarter of the way and you will get a trickle. Open it three-quarters and the material exits fast enough to pile well clear of the tub side.
The 7010 VT uses a bolt-on door frame design, which matters more than it sounds. All the cylinder mounts and actuation hardware live on the door frame itself, not on the tub sidewall. If the door catches a low bunk edge or gets forgotten in the down position at the end of a run, the bolt-on frame takes the impact and can be unbolted and repaired or replaced without touching the main structure. That is a practical durability decision for a machine that runs multiple passes every day.
Where the geometry really starts to matter is in how the auger’s rotation interacts with the opening. As the auger turns, material is expelled in a pulsing pattern rather than a steady stream. A wider or taller door opening smooths this somewhat, but you will still see slight variation in pile density through each auger revolution. This is not a defect; it is the nature of vertical auger discharge. What you can control is whether that variation compounds into a meaningful difference along the bunk, or stays small enough not to matter.
Linking Ground Speed to Pile Shape
Most operators have feathered the door mid-run at least once. You are about two-thirds of the way down the bunk, you can see the feed is stacking a little deep, and you open the door another notch. The problem is that the remaining bunk length now gets a different application rate than the section you already covered. By the time you finish, you have three zones: a correctly fed section, a light section where you were still figuring it out, and a heavy section where you over-corrected.
The better approach is to set the door height and leave it. Let tractor ground speed do the adjusting. The relationship is straightforward: faster ground speed with the same door opening produces a thinner, more spread-out windrow. Slower speed thickens it. If you determine your correct door height off the bunk first, by driving alongside an empty area and observing the fall rate at the speed you intend to travel, you arrive at the bunk with a calibrated starting point. Then speed becomes your only variable.
For rations with significant chopped roughage or longer-stemmed material, this matters even more. Those rations have more air in them and will stack differently to a finer, wetter blend. A coarser ration at the same door height and speed will produce a taller, looser pile. You need to either increase speed slightly or reduce door height to compensate. Neither is wrong; they are just different tools for the same result.
This same principle of coordinating discharge variables runs through how auger design affects the consistency of material leaving the tub in the first place. If the mix quality entering the door is uneven, no amount of door adjustment will fix what lands in the bunk.
Common Fault Patterns: One Side Rich, the Other Short
If one side of your bunk runs consistently heavier and the other consistently lighter across multiple batches, the door setting is rarely the first place to look. The more common causes fall into a short list.
- Turning at different speeds: Many operators run a slightly different ground speed on the return pass compared to the first pass. The bunk line looks even at the midpoint but the two ends tell a different story. Measure your speed at both ends of the run.
- Door opening drift: On machines with worn or poorly adjusted actuator linkages, the door can slowly creep open during the run as hydraulic pressure backs off. What started as a 30 cm opening finishes at 40 cm. The feed line progressively thickens toward the end of the bunk.
- Uneven mix density in the tub: If wet silage settled to one side of the tub during loading, the early part of your discharge run will carry denser, heavier material. You may attribute this to door drift when the problem actually started before you opened the gate.
- Auger speed inconsistency: PTO speed affects how aggressively material is thrown toward the door. If engine RPM drops mid-run because of a rise in ground terrain or a tight turning radius, the discharge pulse rate changes. Keep PTO RPM consistent throughout the bunk run.
An operator in the Free State running a mid-size dairy on a multi-pen layout described a persistent rich-to-light pattern on one run that turned out to be nothing more than a slight downhill grade over the last 20 metres of bunk. The tractor would accelerate slightly on the grade without the driver noticing, thinning the feed line right where the milking cows stood. A fixed gear rather than a hand-throttle-managed speed fixed it within a day.
The Relationship Between Auger Speed and Discharge Consistency
Auger speed determines not just how much material moves toward the door, but how uniformly it is presented to the opening. At lower auger speeds, material arrives at the door in more distinct pulses, producing a lumpier windrow. At higher speeds, the flow is steadier but the power draw increases and finer, lighter ingredients can get thrown past the bunk edge in a crosswind.
There is no universal correct speed. The right auger speed depends on ration moisture, ingredient particle size, and discharge door opening. The practical approach is to treat auger speed as a setting you establish once per ration type, not something you vary during a run. Change the ration, revisit the speed. Keep it stable within a ration.
For operators running multiple ration types daily, documenting these combinations saves time. A simple logbook entry of door height, ground speed, and PTO RPM per ration type eliminates the guesswork at the start of each batch. It also makes it easier to hand the task to a second driver without the bunk line quality changing. LEO Agriculture’s VT Series mixers include ISOBUS-compatible weighing as a standard feature, and integrating discharge records into that same data habit keeps everything in one place.
The Bolt-On Frame: A Practical Note on Daily Operations
Discharge door damage is common enough that it warrants a design response. Forgetting the door is in the down position at the end of a run, catching a bunk divider, or backing up against a wall with the door open all happen on working farms. The bolt-on frame design on the 7010 means the damaged component is a replaceable part rather than a structural repair. All actuation hardware mounts to the frame, so removing and refitting it does not require welding or major disassembly. For an operation running two or three batches a day through a KwaZulu-Natal feedlot or a Western Cape dairy, the difference between a two-hour frame swap and a week waiting for a fabrication job is not trivial.
Worth noting: the bolt-on door frame concept is also relevant when thinking about how smaller mixer configurations handle wear and impact events differently from larger machines, since discharge geometry scales with tub volume and the door dimensions change accordingly.
Once you have a stable door height set for each ration, a consistent ground speed for each pen, and your PTO speed locked in, the bunk line problem largely solves itself. The cows sort it out from there.
