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Two post and four post lifts (to ensure the safe lifting of a vehicle) are required to have an automatic locking system built into each the columns. To release these locks, most lift manufacturers offer either a 2-post single point release or a 2-post dual point release on their two post lifts. Four post lifts may have a variety of lock release methods. Certain types of four post lock release designs include a 4-post manual single point release design, or an 4-post air operated single point release design.
When the lift arms or runways are raised, the automatic locking mechanisms are engaged at different height intervals. The operator will hear a distinctive "metal on metal sound" as the locks become engaged into the locking bar on the inside of the columns. These locking positions are normally located at between two and four inch intervals.
1. If the lift is being raised and the hydraulic system would fail, then the carriage (vehicle) would descend ONLY to the next lower locking position. The column locks would automatically engage to prevent the vehicle from descending past this position.
2. When lifting a vehicle, the lift arms or runways are raised to the desired height, and should then be lowered slightly to the next lowest lock position. The column locks act much like a pair of jack stands. If you are lifting a vehicle with a floor jack, you would jack the vehicle to the desired height and then place jacks stands under the lift points. Neither the hydraulic system in a floor jack nor the hydraulic system in an above ground lift is designed to support a vehicle. The hydraulic systems in both the floor jack and the above ground are designed to "raise" the vehicle-not support it. You must use jack stands if you are using a floor jack, and you must use the safety locks on an above ground lift to ensure safety.
When the vehicle is ready to be lowered, the safety locks need to be disengaged. This procedure can be accomplished in two different ways (for the two post above ground lift). First, the vehicle needs to be raised slightly to disengage the locks. Then, if the lift is equipped with single point release, the release lever (next to the power unit) is pulled downward to release the locking mechanism on both columns simultaneously. The operator can then lower the lift by pulling down on the dump valve lever while holding the lock lever down in the other hand. The spring loaded single point release lever needs to be "held down" during the entire lowering process. If the single point release lever is not "held down" during the descent, then the locks (in both columns) are automatically re-engaged, so the lift will descend ONLY to the next lower lock position. If the lever is released, then the locks will also re-engage, allowing the vehicle to be raised back to the desired height.
Some over head two post lifts (and most base plate lifts) have a dual point release system. This system requires the operator to manually release the locks from both columns by pulling a cable or small bar attached to the column's locking mechanism. Although this system requires the operator to walk to the other column to release the locks, the lock system on the dual point release system is designed to function just as effectively as the lock system on the single point release system.
The basic disadvantage of the dual point release system is the time involved for the operator to walk from one column to the other column and then back again. Most COMMERCIAL GRADE over head two post lifts DO NOT incorporate the dual point release system.
Most base plate lifts have the dual point release system because the cable needed to actuate the single point release system works best when run through the over head beam found only on the over head two post lifts.