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A lift valve is a control device used to apply air to a semi-tractor's lift or tag axle, commonly called a drop axle. Typically located on the tractor's dashboard, the lift valve usually is accompanied by an air pressure gauge that indicates the amount of air pressure being applied to the tag axle. When a driver feels the situation warrants the tag axle being engaged, the lift valve is actuated to apply air to the axle. Once the driver is confident that the use of the tag axle is no longer required, the air can be bled off the air suspension by switching the lift valve off.
Semi-tractors use a tag or drop axle to meet certain load weight restrictions by spreading the weight of the loaded tractor and trailer out over a wider area. By adding a drop axle to a tractor or trailer, the driver has the ability to actuate the air suspended axle with the lift valve. This applies air to an air bladder, which pushes the axle down onto the road. By manipulating the amount of air pressure being pumped into the air bag suspension, the driver has the ability to alter the way the vehicle distributes its weight onto the roadway.
When the semi-truck passes over a weigh station or scales booth, the booth operator will inform the driver if the vehicle is within compliance as to the weight that is being transported. Occasionally, the driver may need to adjust the location of the trailer wheels or the fifth wheel or drop the tag axle to reallocate the weight to be in compliance. One problem with placing many tires on the ground is that the vehicle will get worse fuel mileage from having to turn the extra tires. The added rubber on the road also makes turning the truck very sharply more difficult due to having to drag the extra tires across the road's surface as it turns.
By using a lift valve, the driver can lift the drop axle in the event of a turn. The driver will also raise the extra wheels and axle in order to negotiate city traffic and backing into a dock. Many drivers will manipulate the lift valve and raise the tires off of the road when driving on the interstate to obtain better fuel mileage. Some vehicles utilize separate lift valve controls for the tractor and the trailer to allow axles to be lowered and raised independently of each other.