What’s the difference, and when does it matter most?
Two-Post lift manufacturers use two primary methods to synchronize lifting carriages and keep vehicles level from side-to-side. These methods include (1) mechanical cable/wire rope equalization, and (2) hydraulic equalization. When lifting heavy vehicles with uneven weight distribution from side-to-side—such as those with cranes or welding units—one method is superior.
That superiority has everything to do with Laws of Physics, specifically Pascal’s Principle.
As we all learned in 7th grade Physics Class, liquids don’t compress easily. Accordingly, lifts equalized using hydraulic fluid maintain an even, level rise despite vehicle weight disparities. Comparatively, cable-driven lifts leverage cable tension to maintain equalization. If one cable stretches, becomes loose, or is installed incorrectly, then the lift does not rise evenly. This puts the operator and—less importantly—the vehicle at risk. [Figure 1]
Manufacturers of cable-equalized lifts note in their Operating Manuals that: (a) cables stretch and fray, (b) daily inspections are required to monitor the integrity of the cable, and (c) weekly or monthly maintenance is required to adjust cable tension to ensure the left and right carriages rise at equal speeds. [Figure 2]
However, as any experienced shop manager or tech will tell you, few perform these inspections and adjustments—they are too hard and time-intensive to perform (especially with cables 16 feet in the air), and are overlooked in the face of more pressing shop matters.
When lifting trucks with uneven weight distribution from side-to-side, a lift will experience unequal stresses on main-side and offside carriage cables. Especially if the cables are already out of sync and not in tension—which is the case more often than not—these side-to-side weight disparities will accelerate cable stretching. Just think of dropping a weight attached to a rope: once the rope is taught, extreme force (due to the inertia of the falling weight) is applied to the rope and that which is holding it. The same concept is true for cables which are out of sync, are not taught, and therefore only “catch” the weight once the cables are back in tension.
Taking these concepts one step further, lift manufacturers also note on their brochures that “Vehicle axle weight MUST NOT EXCEED one half of total lift capacity.” This is 100% accurate. What is not stated, however, is that the same concept is true for the vehicle’s side-to-side weight rating. This side-to-side weight differential taxes both the structural integrity of the lifts arms and carriage (impetus for the Axle Weight warning above), and also the Equalization system employed by the lift.
Choosing which lift is best suited to service vehicles with both a front-to-rear and side-to-side weight differentials is inherently more complicated than the manufacturers note above would make it seem. Of course, side-to-side weight disparities most frequently occur when a front-to-rear weight disparity also exists. You may be surprised how quickly a 3,000lb side-to-side weight disparity from a Right-Side mounted Welding Unit stretches and puts a cable-equalized lifting system out of sync (and wears down plastic slide blocks).
With what we know about a cable’s tendency to stretch (as noted directly in the manufacturer operating manuals), many have concluded that cable-driven lifts are not the appropriate equipment for lifting vehicles with significant side-to-side weight disparity. A full hydraulic system—as opposed to a stretchable, adjustable, variable length wire rope / cable equalization system—will ensure the safest, most level rise. And, with technicians being asked to do more than ever, a hydraulic system will keep them focused on tightening bolts on the vehicles in your shop, not the carriage bolts on your lifts.
A simple concept we all learned in 7th Grade Physics is indisputable, even in a shop environment which may at times defy the truths we all hold dear.