By duty cycle and by whether the load must hold itself.

A machine screw jack is inefficient because its thread slides in the nut, and that friction becomes heat. On intermittent duty that is irrelevant, and the friction delivers a genuine benefit: the jack is normally self-locking, so it holds the load when the drive stops without a brake.

A ball screw jack rolls instead of sliding. Efficiency is much higher, so a far smaller motor moves the same load and very little heat is generated - which is what makes continuous or high-duty-cycle operation possible. But it will back-drive under load, so a brake is mandatory on anything that must hold position, and on a vertical load that brake is a safety component.

The practical rule: intermittent lifting and long holding periods point to machine screw; frequent or continuous positioning points to ball screw with a properly specified brake.

It is useful, and it is not a substitute for a brake or a safety device.

Self-locking comes from friction in the screw thread, and friction is not a dependable quantity. It falls as the jack warms, it changes as the nut wears and the lubricant ages, and vibration can allow slow creep that would not occur in a static test. A jack that holds perfectly on commissioning may creep years later.

Manufacturers are generally explicit that self-locking should not be relied upon as the sole means of holding a load, particularly where people can be beneath it.

Where a suspended load, personnel access or a safety function is involved, fit a brake on the drive and consider a safety nut. Use the self-locking property to simplify normal operation, not to satisfy the safety case - which is the same distinction that applies to worm gearboxes generally.

A second nut that carries no load in normal service and takes over if the main nut fails - required wherever failure would drop a load.

The lifting nut in a machine screw jack is bronze and wears against the steel screw over its life. If it wears through completely the load drops, and on a vertical jack that is a catastrophic failure.

A safety nut sits behind the main nut with a small clearance. It carries nothing while the main nut is sound. As the main nut wears, the gap closes, and there is normally a visible indicator showing that wear has reached the point where replacement is due. If the main nut fails entirely, the safety nut catches the load.

It is a wear-indicating and fail-safe device rather than a permanent second load path, so a jack running on its safety nut must be taken out of service.

Specify one on any vertical lifting duty, anywhere people work beneath the load, and wherever inspection access is limited.

Mechanically by line shafts and bevel gearboxes, or electronically with servo drives and position feedback - and mechanical linking is the more robust.

Many applications lift a platform on three, four or more jacks that must stay level. A mechanical system connects them with shafts and right-angle gearboxes driven from one motor, so the jacks turn in fixed proportion. Synchronisation is guaranteed by the linkage: it cannot drift, and it does not depend on any control system working.

An electronic system drives each jack with its own servo motor and encoder, with a controller keeping them in step. It is far more flexible, allows deliberate non-level motion, and avoids long shaft runs - but synchronisation now depends on the control system and its feedback remaining correct.

Mechanical linking is generally preferred where a loss of synchronisation would rack the structure or drop a load. Electronic control suits complex or reconfigurable motion where that risk is managed.

On load, stroke, duty cycle and speed together - with duty cycle usually the constraint that surprises people.

The static load capacity is the obvious figure, and it must include the worst case with a safety factor, plus any side load or moment the jack will see.

Stroke determines screw length, and a long screw in compression must be checked for buckling - a jack adequate on load capacity can fail by buckling if the extended screw is long and slender. Loading in tension avoids this entirely, which is why some installations are arranged to pull rather than push.

Duty cycle governs heat. A machine screw jack turns most of its input into friction, and running one continuously will overheat it. Manufacturers publish a duty cycle - a permitted percentage of running time - and exceeding it is a common cause of failure.

Speed is limited by the same heat and by critical speed of the screw at long strokes.

Grease on the lifting screw and in the worm gear housing, checked and replenished on schedule - and nut wear monitored.

The lifting screw is exposed on the extended portion and must be kept greased along its working length. That grease also collects dust and grit in dirty environments, and contaminated grease is abrasive - so in those conditions bellows boots to protect the screw are worth specifying, and the grease should be cleaned off and replaced rather than simply topped up.

The worm gear housing holds grease or oil to its own schedule.

Nut wear is the main condition indicator on a machine screw jack. Where a safety nut with a wear indicator is fitted, checking it is straightforward. Without one, wear is measured as backlash between the screw and nut, and manufacturers publish the limit.

A jack that has become noisy, or that shows increasing backlash under reversing load, is reporting nut wear.

Yes, and both are common - but the lubrication arrangement and sometimes the model must suit the orientation.

Jacks are frequently used horizontally for pushing and pulling, for positioning gates and dampers, and for adjusting machine elements. Inverted and upside-down mounting is also normal.

What changes is where the lubricant sits. A housing designed for upright operation may not keep its worm gear set lubricated when inverted, and grease on the lifting screw will migrate differently. Manufacturers specify which orientations a given model supports and what to change - sometimes a different fill quantity, sometimes a different model.

Protection also changes. In an upright jack the screw enters the housing from above and debris tends to fall away; inverted, it can fall in. Boots and covers become more important.

State the orientation when ordering rather than assuming a jack is orientation-neutral.