When lifting a car off the ground, you must always remember that the laws of gravity apply: this means that the vehicle will take advantage of every opportunity to come back down, and hence the mechanism holding it up must be completely sturdy for that vehicle's weight.  

Car lift systems aren't created equal; there are many different types and different models even within those types. For every model you're considering, however, the following are non-negotiable features to ensure safety once the system is installed and in use:

1. Make sure the lift has a slack safety device

A slack safety device, also known as a slack safety lock or slack-cable safety latch is the mechanical locking pawl which prevents the lift from coming back down once the loaded lift has been raised to the desired position. It consists of a sturdy metal tongue with springs, designed to slide into holes while the car is being raised. The pawls are released by holding down a lever while the lift is being lowered – at this point, only the hydraulic piston and cabling prevent the car from slamming down. The slack safety device therefore provides additional safety by locking the lifting cable automatically, preventing the disaster that would surely come if the cabling came loose for any reason e.g. when a pulley breaks.

2. Safety locks should be adjustable

In a car lift, the hydraulic cylinder is only responsible for moving the car up and down; it is the pawls sliding into holes on the posts that actually bear the weight of the load once the vehicle has been raised. For residential garages, you probably have a gentle slope on the floor to facilitate drainage – usually a few inches' drop between the farthest wall and external door.

When the lift is being assembled – whether by you or professionally – ensure that the holes into which locking pawls fit are at the same level, especially if installing four-poster lifts. If not, one side's pawls will engage before the others causing the deck to sag slightly. If the car's emergency brake isn't engaged or the car isn't secured in any other way, this cause the car to roll out from the deck. Look for lift-makers who offer supply shims or screw adjusters which facilitate these minor adjustments.

3. Sheaves should be self-lubricating or externally lubricated

The car lift cabling runs through the sheaves, and they support a very heavy load when the lift is in use. Cheaper car lift models don't have bearings or bushings in the sheaves, which increases the wear on them causing galling and premature failure. Choose lifts which larger sheaves as these turn slower and hence sustain less wear. In addition, they should run on bushings or bearings.

Bonus tip: Trust the brand

It's not uncommon to see lift brands that are here today and gone tomorrow. For something as sensitive as a car lift, you want to go with renowned brands that have been around for a long time. This is not only for your warranty's sake, but also to make sourcing for replacement parts much easier.