What Is Drift in Mineral Processing? A Field Checklist for McLanahan Equipment

What is drift?

I work on the response side of mineral processing—the side that gets called after a plant has lost production but no mechanical fault has been found. Drift is usually part of that story. Drift is the slow movement of an operating condition away from the range where the equipment was intended to run.

What is drift? It depends on who you are talking to. An instrumentation person will say it is a sensor reading that changes over time. A process person will say it is the product getting worse even though the control screen looks normal. In the plant, I use the word in a broader sense: a gradual change that makes the machine less predictable. It creeps instead of crashing.

From the outside, a drifting machine still looks busy. The screw is turning. The feeder breaker is breaking. The filter press is making a cake. What you do not see is the extra adjustment, the higher motor amps, or the borderline quality at the end of the shift.

How drift shows up in McLanahan equipment

McLanahan sand screws, feeder breakers, and filter presses are heavy-duty machines, but they all depend on a consistent operating point. When drift appears, it tends to show up in the same patterns:

• Sand screw: motor amps slowly climb, the overflow gets cloudier, or the screw needs more frequent adjustment to keep the same product.
• Feeder breaker: crusher amps or hydraulic torque start drifting upward because feed conditions changed or because breaker teeth are wearing faster than normal.
• Filter press: cycle time becomes longer, cake moisture drifts out of spec, or the press reaches target pressure only after repeated corrections.

In my experience, drift rarely has one simple cause. It is usually a combination of small changes: a feed source moves, an instrument drifts, an operator adjusts a setpoint, and a wearing part finally makes the machine cross a threshold.

Use this drift checklist when...

• Production rate is down but no alarms are active.
• The same machine now needs manual adjustments every shift.
• Product quality is only slightly outside target, not dramatically off.
• Maintenance replaced parts, but the problem came back within days or weeks.

The goal is to find the system condition that changed, not just to replace the part that looks worn.

1. Start with the 48-hour trend, not the machine

Before pulling a grease gun or opening a panel, look at the last two days of operating data. You are hunting for when the drift started, because that moment usually contains the cause.

Check trends for:

• Feed rate
• Motor amps
• Pressure and temperature
• Level and density
• Setpoint changes

If there is no trend historian, start one. A drift problem without a trend line is just a guess with a wrench nearby.

2. Confirm the setpoints did not move

Here is the thing: people start investigating machine drift and overlook setpoint drift. Someone on the previous shift may have changed the target speed or pressure to get through a bad feed condition, and the change was never written in the log.

I have seen plants chase a mechanical problem for two shifts when the real answer was a five percent speed change made by an operator trying to protect the process. The equipment was not drifting. The instruction was different, and the machine was doing exactly what it was told to do.

Verify the setpoints against the original operating procedure, not against what you remember from last week.

3. Check the real feed conditions

Drift can ride in with the feed. If the crusher product changes size, if a stockpile starts to segregate, or if slurry density shifts, the downstream equipment will act like it is drifting even though nothing on the machine has changed.

Most operators focus on the machine reading and completely miss the feed condition. Ask a better question: what changed upstream before the drift started?

Check feed moisture, feed size distribution, percent solids, and recirculating load. If a McLanahan sand screw is suddenly drawing more amps, the first question is usually not about the screw. It is about the amount and shape of solids being fed into it.

4. Verify the measurements before blaming mechanics

Instrument drift is one of the most common kinds of drift, but it is also the easiest to test. If a pressure transmitter drifts high, the control system reacts to a pressure that is not really there. If it drifts low, you do not find out until another limit has been exceeded.

Before replacing a valve, cylinder, or drive component, compare the control room reading with a local gauge or a hand-held test instrument. Check the sensor line for blockage, moisture, or trapped air. Verify the zero reading. Clean the face if needed.

The question everyone asks is, Is the machine drifting? The question they should ask is, Is the instrument telling the truth?

5. Inspect the mechanical parts that carry the load

If feed and measurements are stable, the next step is mechanical drift. This is the kind most maintenance teams expect first, but it should not be checked first.

For a McLanahan fine material screw washer, check flighting and paddle wear, bearing clearances, and drive alignment. Worn flighting can let material slip backward and push motor amps up even when feed has not changed.

For a feeder breaker, inspect breaker teeth or picks, drum clearances, and conveyor chain condition. A small amount of uneven wear can change the machine load more than people expect.

For a filter press, look at filter cloths, drain ports, feed passages, and plate surfaces. A cloth can look clean on the outside while the inner layer is already blinded.

6. Write down the decision, not just the work order

The fix only lasts if the next person understands why the machine drifted in the first place. A work order that says adjusted feeder speed is not enough. Write down:

• What changed before the drift started
• Which setpoint changed and why
• What the feed or slurry density was at the time
• Which instrument was checked and how
• Which mechanical wear items were inspected

That kind of record stops the next crew from replacing a good part for no reason.

Common mistakes when chasing drift

Changing parts before checking the process

Drift does not always mean broken. I still kick myself for the times I let an aggressive maintenance plan push me toward replacement parts before checking feed history and setpoint changes. The equipment was not failing. It was responding to a change in what it was being asked to do.

Treating drift as only an instrumentation problem

Sensor drift is real, but it is not the only kind of drift. Process drift and mechanical drift follow the same pattern, so keep the checklist broad until the evidence narrows it down.

Looking at the machine instead of the whole system

Drift is often a chain reaction. A small change upstream makes the sand screw work harder, which increases wear, which changes the next process step. If you only inspect one piece of equipment, you can fix the symptom while missing the cause.

There is something satisfying about finding drift before it becomes an emergency. It is not the kind of fix that gets applause. It is the quieter win of seeing the trend line flatten again after days of guesswork.

That is the best outcome: the machine runs without someone standing next to it, waiting for the next drift.