A scissor lift table rated for 1 ton stopped lifting. The motor ran, the gearbox turned, the screw rotated, and the table went up about 30 mm, then stalled. The customer checked the motor — fine. Checked the gearbox — fine. Checked the screw — visually fine. The lift had worked for a year, then gradually got slower, then stalled. The customer assumed the motor was undersized and ordered a bigger one. The bigger motor also stalled. The problem was the screw jack efficiency, and it had collapsed without any visible damage.

A screw jack is a sliding screw. The nut rides on the thread, and the load is carried by the thread surfaces rubbing against each other. The efficiency of that sliding contact depends entirely on the friction between screw and nut. A well-lubricated steel screw on bronze nut runs at 30-40% efficiency. A dry or worn screw drops to 15% or lower. The input torque needed goes up by the same factor. A screw that ran at 35% efficiency and now runs at 15% needs 2.3 times the input torque. That is exactly why the lift stalled.

The efficiency math

The torque required to lift a load with a screw:

T = (F × dm / 2) × (tan(λ) + μ) / (1 − μ × tan(λ))

Where F is the load force, dm is the mean thread diameter, λ is the lead angle, and μ is the friction coefficient. The lead angle for a typical machine screw jack is 2-5 degrees. The friction coefficient for a lubricated steel-bronze pair is 0.08-0.12. The efficiency is:

η = tan(λ) / ( (tan(λ) + μ) / (1 − μ × tan(λ)) )

For λ = 4° and μ = 0.10: η ≈ 0.37. For the same screw with μ = 0.30 (dry, or worn bronze with embedded grit): η ≈ 0.16. The input torque more than doubles. The 1.5 kW motor that lifted the table at full load now sees a 3.5 kW equivalent load. It stalls at 30 mm of travel.

Why the efficiency collapsed silently

The lift was used in a dusty environment — a metal fabrication shop. Over a year, the dust worked into the screw thread. The nut was a standard bronze nut with a grease nipple, but the nipple was never serviced. The grease dried out. The screw ran dry with embedded grit acting as lapping compound. The bronze nut wore, and the wear particles mixed with the dust to form an abrasive paste. The friction coefficient climbed from 0.10 to over 0.30. Nothing looked wrong from outside — the screw still turned, the nut still engaged. The efficiency was gone.

The visible clues were there, but nobody looked: the nut was warm after a lift cycle (friction heat), the screw showed a faint polished wear band, and the grease nipple was plugged with hardened grease. On a healthy jack, the nut runs cool and the screw shows a thin uniform oil film.

The fix and the upgrade

The customer replaced the nut (a $45 bronze nut), cleaned and re-greased the screw with lithium grease, and the lift worked again at full load. But the real fix was a service schedule: grease the screw monthly, inspect the nut wear annually, and replace the nut at 50% thread wear. The stall had cost a day of downtime and nearly bought a new motor that was never needed.

The upgrade option for the same application: switch the bronze nut to a recirculating ball nut. A ball screw jack runs at 85-90% efficiency instead of 35%. The input torque drops by a factor of 2.5. The same motor now lifts 2.5 tons instead of 1. The ball nut costs more (about 3x the bronze nut) but it lasts longer and does not degrade with dust the same way. For machines that run in dirty environments or cycle frequently, the ball screw jack pays for itself.

The tradeoff: a ball screw is not self-locking. A bronze screw jack holds its position when the motor stops — the friction holds the load. A ball screw back-drives. If the motor releases, the table falls. For a scissor lift, that means a brake or a counterbalance valve is mandatory with a ball screw. The bronze jack needed no such device. This is the classic tradeoff: efficiency and wear against self-locking and simplicity.

When the jack is undersized from day one

There is a second, more common version of this problem: the jack was undersized when it was installed. The catalog rating for a screw jack assumes 25% efficiency and a 30-second duty cycle. The fabrication shop lift was used for continuous indexing, 20 cycles per hour, all day. The catalog rating does not apply at that duty. The screw heated up, the grease thinned, the friction rose, the efficiency dropped, and the stall appeared after a year.

The sizing rules for screw jacks that hold up:

  • Take the catalog capacity and derate by 50% for continuous duty.
  • Check the input torque at the actual load, not the catalog torque. The catalog assumes 25-30% efficiency.
  • Check the screw critical speed for the travel length. A long screw at high RPM whips.
  • Check the column buckling for the load and unsupported length.
  • Size the motor for the worst case: cold start, dry screw, 15% efficiency.

The last rule is the one that would have caught the fabrication shop problem. If the motor was sized for 15% efficiency instead of 35%, the lift would have worked through the degradation. Instead it stalled at the worst possible moment, mid-cycle, with a load on the table.

The check that takes ten minutes

Every screw jack installation should have a torque baseline. Measure the input torque at no load and at full load when the machine is new. Record the number. Once a quarter, repeat the measurement. If the full-load torque has risen 50% from baseline, the screw is degrading. Service it before it stalls. The measurement is a torque wrench on the input shaft. It takes ten minutes and it catches the exact failure that stopped the scissor lift.

A screw jack loses efficiency silently — dust, dried grease, and worn bronze push the friction up and the input torque with it. The lift that stalled at 1 ton was not undersized at install. It was running at 15% efficiency instead of 35%. Grease the screw, watch the nut wear, and baseline the input torque. A ball screw jack fixes the efficiency but brings back-drive with it.