An increasing number of today's liftgates are equipped with power-down pump systems instead of gravity-down pumps. Power-down pump units have grown in popularity because they provide more consistent lowering speeds, regardless of the weight of the liftgate platform or weather conditions.

Power-down pumps use hydraulic force to both raise and lower the liftgate. By contrast, gravity-down pumps use hydraulic force only to raise the liftgate platform, relying on the weight of the liftgate and an electronically activated valve to lower the platform. This may result in less consistent speeds, when compared with the use of hydraulic force.

A power-down hydraulic system works in a closed-loop fashion. The piston inside the cylinder forces the exchange of hydraulic fluid from one side of the piston to the other, through the pump reservoir. With power to the pump, fluid is forced from one side of the piston into the reservoir then is pumped out of the reservoir to the other side of the piston with no overflow. If power to the pump is interrupted and the operator activates the lowering function, then fluid is forced from one side of the piston back into the reservoir.

However, due to lack of power, the fluid is not able to be pumped back to the other side of the cylinder, causing excess fluid to collect in the reservoir. Once the reservoir capacity is reached, excess fluid overflows through the reservoir breather cap. The following questions and answers may help you understand when and why this occurs.

Q: Why does my power-down pump occasionally experience fluid overflow during the lowering phase?

A: You may be experiencing a power loss to the pump motor. During a power loss, the pump unit cannot pump the fluid from
one end of the cylinder to the other, so all of the fluid gathers in the reservoir. Because the pump reservoir is not large enough to contain all of the fluid in the system, an overflow occurs.

Q: Why not just make a larger pump reservoir?

A: There are two reasons. One, space constraints do not allow for a larger pump reservoir. Two, we have found that most customers fill their reservoir to the top. If we made the reservoir larger, and then people filled it to the top, it would overflow when the pump is operated because there would be nowhere for excess fluid to go.

Q: My pump is equipped with a Thermalpak® with VoltageGuard™ controller. Why doesn't this prevent the gate from lowering when it shuts down, so that there would be no overflow?

A: Thermalpak with VoltageGuard shuts the pump down when it senses any danger of overheating or low voltage. This protects the liftgate motor from potentially serious damage. However, our engineers purposely made it possible for an operator to manually lower the liftgate in the event the pump shut off in order to lower the cargo to a secure position on the ground. While this may result in a fluid overflow, we believe the need to secure the cargo is more important than preventing an overflow.

Q: How do I know when this situation has occurred?

A: Thermalpak with VoltageGuard usually shuts down the liftgate during the lift cycle. If the gate stops during lifting, the operator's normal reaction is so see if he can lower the gate. If you cannot hear the pump running while you are trying to lower the gate, then you will know that there is no power running to the motor. At this point, an overflow may occur if you lower the gate significantly. Pumps equipped with Thermalpak with VoltageGuard have the added advantage of an alarm that sounds if you try to raise the gate when the pump is shut down. This serves as a warning that you may cause an overflow if you lower the gate significantly. For questions regarding your power-down pump unit, contact your Waltco account representative at 800-211-3074.


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