One of our colleagues shared an issue encountered at the workplace involving a hydraulic machine:

The setup comprises Car Lift Repair Tampa Florida vane pumps positioned atop the reservoir alongside suction strainers. The issue arises intermittently, wherein one of the three pumps fails to prime properly. Once primed, the machine functions as expected. However, after breaks or lunch breaks, upon restart, a pump occasionally fails to prime. Any insights would be appreciated.”

Ideally, it would be preferable to avoid mounting the pumps on top of the reservoir, as a flooded inlet is optimal for hydraulic pumps, and to eliminate suction strainers, although this may not directly address the problem. Changing the mounting position of the pumps presents a more challenging task.

Unlike electric motors, Car Lift Repair Tampa Florida vane pumps rely on centrifugal force to engage the vanes with the cam ring. Running the vane pump above its minimum recommended drive speed, typically around 600 RPM, is intended to ensure proper vane engagement during startup.

Consequently, conventional wisdom dictates that outlet pressure is unnecessary for priming a vane pump. The centrifugal force on the vanes establishes the initial seal, enabling the pump to draw fluid from the reservoir. For instance, Rexroth’s PV7 vane pumps literature explicitly states: “All PV7 vane pumps are self-priming.”

My experience, corroborated by the issue raised by this colleague, suggests that vane pumps, especially when mounted above the tank, are prone to becoming ‘air bound’, even when air downstream of the pump outlet can freely return to the tank via an open center directional control valve.

Whenever I’ve encountered this problem previously, configuring the outlet plumbing to create a head of oil above the pump, even if just a few inches, has resolved the issue.

While the head of oil may not significantly affect the vanes contacting the cam ring, it’s plausible that arranging the plumbing in this manner ensures the pump fills itself with oil, allowing it to displace or even compress any air in its outlet line.

It’s worth noting that external gear pumps are also susceptible to becoming ‘air bound’, even though there are no issues concerning vane throw against the cam ring in their case. Similar to vane pumps, pressure-balanced, high-efficiency gear pumps do not handle air well, despite being generally considered “self-priming.”

Running the gears without a continuous oil supply leads to metal-to-metal contact between critical parts in the pump, designed to operate on a hydrodynamic oil film. Although this loss-of-lubrication damage may not be immediately apparent once the pump fills and starts displacing oil, the damage is already incurred.

Therefore, it’s prudent to assume that all “self-priming” Car Lift Repair Tampa Florida hydraulic pumps may not be truly self-priming, especially during initial startup. Whenever feasible, purge the intake line of air and, if appropriate, manually turn the pump (gear-type only) with the outlet vented to ensure it’s not air bound. If safe, bleed the outlet during startup to ensure the pump is displacing oil rather than spinning in an air lock.

Load sensing refers to a method of regulating variable pump output in open circuits. This approach is named for its ability to detect the pressure downstream of an orifice induced by the load, adjusting pump flow to uphold a consistent pressure drop (and thus, flow) across the orifice. Typically, the ‘orifice’ is a directional control valve with proportional flow traits, though depending on the application, a needle valve or a fixed orifice might be utilized.

In Car Lift Repair Tampa Florida hydraulic systems prone to significant fluctuations in flow and pressure, load-sensing setups can result in significant power savings. In systems where all available flow (Q) is consistently converted into useful work, power loss to heat is limited to inefficiencies. However, in systems equipped with fixed displacement pumps where the entirety of available flow is only intermittently needed, excess flow passes through the system relief valve, leading to heat conversion. This issue is exacerbated if the load-induced pressure (p) falls below the relief valve setting, resulting in additional power loss due to pressure drop across the metering orifice (control valve).

Since this type of control regulates Car Lift Repair Tampa Florida pump flow at the maximum pressure setting, power is lost to heat due to the potentially significant pressure drop across the metering orifice.

A Car Lift Repair Tampa Florida load-sensing controlled variable pump, also known as a flow-controlled pump, largely mitigates these inefficiencies. 

Typically, a load-sensing circuit consists of a variable displacement pump, often axial-piston in design, equipped with a load-sensing controller, and a directional control valve featuring an integral load-signal gallery. This gallery (LS, depicted in red) links to the load-signal port (X) on the pump controller. In the directional control valve, the load-signal gallery connects the A and B ports of each control valve section through a sequence of shuttle valves, ensuring that the actuator with the highest load pressure is sensed and feedback is provided to the pump control.

To grasp the coordinated functioning of the load-sensing Car Lift Repair Tampa Florida pump and directional control valve, consider a winch operated via a manually actuated valve. As the operator moves the spool in the directional valve to 20 percent of its stroke, the winch drum rotates at five rpm. In this scenario, envision the directional valve as a fixed orifice. Flow through an orifice decreases as the pressure drop across it diminishes. With increased load on the winch, the load-induced pressure downstream of the orifice (directional valve) rises, reducing the pressure drop across the orifice and consequently slowing down the winch.

In a load-sensing circuit, the load-induced pressure downstream of the orifice (directional valve) is conveyed back to the pump control via the load-signal gallery in the directional control valve. Responding to the elevated load pressure, the load-sensing controller increases pump displacement (flow) slightly to raise the pressure upstream of the orifice by a corresponding amount. This maintains a constant pressure drop across the orifice (directional valve), thereby ensuring consistent flow and, in this scenario, a constant winch speed.