Because they lift very large vehicles without building a very large lift into the floor, and they can be repositioned for any configuration.

A fixed lift capable of raising a bus or an articulated truck is a major installation with a correspondingly large footprint.

Mobile columns are moved into position around whatever vehicle is present, so one set handles a range of wheelbases and axle counts.

They can be moved between bays, taken outside, or transported to another depot.

That flexibility suits a mixed heavy fleet far better than a fixed installation sized for the largest vehicle.

The trade is set-up time for each vehicle, which a drive-on fixed lift does not require.

One per wheel - four for a two-axle vehicle, six or eight for three and four-axle vehicles and articulated combinations.

Sets are sold in fours and can usually be extended, with the control system supporting a maximum number of columns.

A four-column set covers rigid two-axle buses and trucks, which for many fleets is most of the work.

Three-axle vehicles need six, and articulated or four-axle combinations need eight.

Capacity is quoted per column and as a set total, and both matter - a heavily loaded axle can exceed a column's individual rating even within the set total.

Buy for the heaviest and longest vehicle in the fleet, since a set that cannot lift one vehicle type creates a permanent workaround.

The vehicle is racked and can be dropped - which is why the control system monitors every column and stops the set if one deviates.

Each column lifts independently, so they must be coordinated by the control system.

If one rises faster or slower, the vehicle twists between its lifting points, stressing the chassis and risking a fall.

Modern sets monitor each column's position continuously and halt the whole set on any deviation beyond a small tolerance.

Communication is by cable or wireless, and a communication fault must take the set out of service - it is the safety function, not a convenience feature.

Never attempt to operate columns individually to correct a level difference unless the manufacturer's procedure specifically provides for it.

A level, sound surface able to take very high point loads through small wheels - a slab adequate for parking is not automatically adequate for lifting.

Nothing is bolted down, but the entire vehicle weight is concentrated onto the columns' small contact patches.

That point loading can exceed what a slab designed for distributed parking loads will take.

The surface must also be level within the manufacturer's tolerance, since columns cannot compensate for a sloping floor.

Expansion joints, drainage channels and any buried services are all places a column should not stand.

Confirm the slab's capacity against the manufacturer's point load figures before committing, particularly in older premises or outdoors.

Correct positioning on the wheels, mechanical locks engaged, axle stands whenever a wheel comes off, and taking the set out of service on any fault.

Each column's forks must be correctly positioned under the tyre, fully engaged and adjusted for the wheel diameter.

Raise until the mechanical locks can be engaged and lower onto them, exactly as with any lift.

When a wheel is removed, that column is supporting nothing - axle stands or a suitable support must take that corner.

Keep the area clear during raising and lowering, and ensure one person controls the set with a clear view of every column.

Batteries on cordless sets must be maintained and charged; a column that loses power mid-cycle is a problem the whole set has to resolve.