How Does a 2-Post Above Ground Lift Work?
A two post above ground lift is designed to raise cars and trucks for easy access to wheel assemblies and the underside of the vehicle. A two post above ground lift is normally mounted to an existing concrete surface. Minimal concrete depth required for a two post above ground lift is four inches. Thicker concrete is required for two post lifts exceeding a 12,000 LB. capacity.
The lift consists of two upright columns, four adjustable arms attached to a carriage assembly, two hydraulic cylinders, an electric/hydraulic power unit, and a variety of hoses, pulleys, and cables. This is the most common style of two post above ground lift found in the USA. There are several European manufacturers building above ground lifts that use an extruded screw/nut configuration powered by an electric motor. (These are not popular in the USA) This discussion will focus on the more popular electric/hydraulic two post above ground lifts sold in the USA.
The two post above ground lift was developed for automotive facilities that were unable to install in-ground lifts. Many automotive service centers are built over rock (and excavation is impossible or impractical) or buildings located on property with a high ground water table. The two post above ground lifts is also the perfect choice to replace an inoperable in-ground lift, thereby avoiding down-time and the removal/installation mess created with an in-ground lift.
Most two post above ground lifts are powered by an electric/hydraulic power unit.
The power unit is an assembly of the following main component parts:
Electric motor (usually 220 Volt single phase 60 HZ). The preferred electric motor should offer maximum torque while drawing the least amount of amperage. A high horsepower motor is NOT an advantage over a lower horsepower motor delivering the same torque.
Hydraulic pump mounted below the motor assembly (with preset valve pressure limit)
Plastic reservoir for the hydraulic fluid
Dump valve handle to allow hydraulic fluid to flow into the reservoir from the hydraulic system.
When the power button on the electric motor is pressed, the electric/hydraulic power unit delivers pressurized hydraulic fluid (through hydraulic hoses) to the two cylinders (one is located in each of the opposing columns). The pressure of the hydraulic fluid is regulated by the factory preset pump valve.
Our Atlas® lifts (although capable of lifting a weight more than the rated lift capacity) are limited to their rated capacity by the pressure valve located in the electric/hydraulic power unit. This pressure valve is FACTORY installed and adjusted to PROTECT the operator from OVERLOADING the lift. DO NOT ADJUST THE PRESSURE VALVE!
(Important: The weight of the vehicle to be lifted must be evenly distributed on the four arm pads. A 10,000 two post above ground lift will easily lift 10,000 LB. of evenly distributed vehicle weight. That means 2,500 LB. per arm. An unsafe lifting condition would exist if a 10,000 LB. vehicle was positioned so that the rear arms supported 4,000 LB. each.)
The pressurized hydraulic fluid flows to both hydraulic cylinders through the set of hydraulic hoses. These hoses are routed either over the top of the lift (through the overhead beam found on an overhead two post above ground hoist) or on the floor (covered by the diamond plate base plate found on base plate two post above ground lifts). Hydraulic fluid DOES NOT MOVE at the speed of light. The hydraulic cylinder on the "smart" column (the column with the power unit attached) will receive hydraulic fluid before the hydraulic cylinder on the "off" column.
To ensure that both carriages and all four arms move at the same time; a set of equalization cables is installed between the carriages. These equalization cables are routed in the same manner as the hydraulic lines. The equalization cables DO NOT do the lifting of the carriages. The equalization cables are used to make sure that all arms are level and locks inside both columns are engaging simultaneously. Yes, the cables apply pressure and are exerting a "force" on the carriages, but their primary job is to equalize the rise and descent of the lift arms (carriages). The hydraulically pressurized pistons (encased in the twin hydraulic cylinders) are responsible for lifting the vehicle. A few manufacturers use a hydraulic system for equalization. Atlas® believe that the aircraft cable equalization system provides the most positive results and use this system exclusively on all two post above ground lifts.
All Atlas® two post lifts have automatic safety arm locks. These locks are designed to secure the lift arms, once the arm pads have been properly positioned under the vehicle. These automatic arms locks engage once when the vehicle is lifted two inches off the ground, and then automatically disengage when the vehicle is lowered to the ground.