Closed Circuit Hydrostatic Drive
Date Posted:15 February 2017
All MultiOne mini loaders feature a closed circuit hydrostatic drive.
Closed circuit drives provide the most power and efficiency to drive a loader, whether articulated or skid steer.
Other manufacturers of ‘stand-up’ mini loaders use open circuit drives.
Open circuit drives are older technology and haven’t been used on their larger skid steer counterparts since the 70’s.
Let’s take a look at some of the key features differentiating the two systems…
Closed Circuit Hydrostatic Drive
Closed circuit hydrostatic drives work by providing flow from a hydrostatic transmission (hydraulic pump with variable flow) directly to the wheel motors.
There is no valving between the hydraulic pump and the wheel motors. Speed is controlled by increasing the flow of the pump; direction is controlled by the pump flowing forward or reverse. This technology is used in all modern sit down skid steer loaders and articulated loaders, including MultiOne loaders.
Open Circuit Hydrostatic Drive
Open circuit drives are inefficient and have low power in comparison to a closed circuit system. Drives used by Australian-made stand-up mini loaders are open circuit. They work by using flow from a gear pump, diverting it through valves and then onto the wheel motors.
Gear pumps are “fixed displacement”. This means the flow of the pump is always the same per revolution of the engine and cannot be varied. The gear pump flows to a control valve. By pushing a lever forward on the control valve, the flow starts to bleed off to the wheel motors. The more flow that is bled off, the faster the wheel motors will turn. The output of the wheel motors then needs to travel back through the control valve and back to the hydraulic tank.
Bleeding off flow like this creates heat in the hydraulic fluid as it is a restriction to the flow. Heat creates inefficiencies and leads to pressure drops. Not only does heat create inefficiencies, but the hydraulic pump still flows the full amount of hydraulic fluid regardless of whether it is needed or not! Compare this to a Multione loader – if you are not moving, the hydrostatic transmission does not flow.