Danfoss 51C110 variable displacement closed circuit hydrualic motor

The 51C110 is a bent-axis, variable-displacement axial piston motor from the Sauer-Danfoss Series 51 and 51-1 product line. It's built for closed-circuit hydrostatic transmissions, meaning it's designed to work in tandem with a variable displacement pump in a loop where fluid is exchanged directly between pump and motor rather than drawn from and returned to an open reservoir.

The "51" identifies the product series, "C" designates a specific control/configuration variant within that series (Danfoss also offers D and V variants at the same displacement points), and "110" refers to the nominal displacement of 110 cubic centimeters per revolution. The motor uses a tilt-block design with spherical pistons — a layout that lets the swashplate angle change under load, which is exactly what "variable displacement" means in practice.

How a Variable Displacement Closed-Circuit Motor Works

It helps to separate two ideas that get bundled together in the name: "variable displacement" and "closed circuit."

Variable Displacement, Explained

In a fixed displacement motor, the angle between the pistons and the drive shaft is set at manufacture and never changes. Every revolution of the motor uses the same volume of fluid, so torque and speed move together in a fixed relationship dictated by flow and pressure.

A variable displacement motor like the 51C110 changes that angle on the fly. At maximum displacement (full swashplate angle), the motor produces maximum torque at lower speed — useful for starting a load from a stop or climbing a grade. As the control mechanism reduces the angle, displacement drops, and the same flow rate produces higher output speed with less torque. This is functionally similar to shifting gears, except it happens hydraulically and continuously rather than in discrete steps.

Closed-Circuit Operation

In a closed-circuit hydrostatic system, fluid discharged from the motor goes straight back to the pump's inlet rather than draining to a reservoir. A small charge pump keeps the loop topped off and replaces fluid lost to internal leakage, but the bulk of the flow just recirculates between pump and motor.

This matters for a few practical reasons:

Reversing is simple. Because flow direction rather than a directional valve determines motor rotation, reversing the pump swashplate reverses the machine — no separate reversing valve bank required.

Response is faster. Closed loops tend to respond more quickly to control input than comparable open-circuit systems, which is part of why they dominate propel drives on tracked and wheeled equipment.

Contamination control is different. Because the same fluid recirculates continuously, filtration quality and fluid cleanliness matter enormously — more on that below.

Typical Applications

The 51C110 and its Series 51 siblings show up wherever a machine needs a compact, high-power hydrostatic drive with a wide speed range. Common examples include:

Crawler and Track Drive Systems

Dozers, excavators, and other tracked equipment use motors like the 51C110 to drive final drives directly or through a gear reduction, taking advantage of the high starting torque at full displacement to get a heavily loaded track moving.

Wheel and Undercarriage Drives

Wheel motors on rubber-tired construction and forestry machines, and undercarriage drives on rail-adjacent equipment, use the same displacement-ratio advantage to combine low-speed pulling power with a reasonable top travel speed — without a mechanical transmission.

Winch Drives

Winches need very high torque at low speed under load and much lower torque at higher speed when spooling freely. A variable displacement motor handles that spread naturally, which is why Series 51 motors are common in winch packages on forestry and marine equipment.

Agricultural and Forestry Machinery

Combines, forwarders, and similar equipment use these motors in propel circuits and auxiliary drives where duty cycles are heavy, sustained, and often run at high ambient temperatures.

Installation and Operating Considerations

A few things experienced technicians watch closely with this motor family:

Fluid Cleanliness and Filtration

Because the motor operates in a closed loop at pressures up to 510 bar, fine internal clearances are doing a lot of sealing work. Contaminated fluid accelerates wear on the piston-to-bore fit and the valve plate, which is usually where displacement control problems and efficiency loss start to show up. Return-line or suction filtration with a reasonably high beta ratio is standard practice, and mixing incompatible hydraulic fluids should be avoided entirely.

Case Drain and Housing Fill

Like most axial piston motors, the 51C110's housing must remain filled with hydraulic fluid at all times, regardless of mounting orientation, to keep internal components lubricated and to manage case pressure. A dry startup after a filter or hose change is a common — and avoidable — cause of premature motor failure.

Thermal Limits

High-pressure, high-cycle applications generate heat, and the motor's temperature rating is based on conditions at the hottest internal point, typically near the case drain. Undersized heat exchangers or restricted case drain lines are a frequent root cause when a motor that "should" be fine keeps overheating.

Matching Control Type to Application

Because Series 51 motors are available with pressure-compensated controls, electric-hydraulic two-position controls, and proportional controls (among others), swapping in a replacement motor isn't just about matching displacement — the control code has to match the system's control logic, or the machine won't behave the way the operator expects.

For more information on the 51C110 hydraulic motor, please visit https://www.baolilaihydraulic.com/product/.

Specifications

Selection Table

Q1: Is the 51C110 an open-circuit or closed-circuit motor?

A1: Closed-circuit. It's designed to work with a matching variable-displacement pump in a hydrostatic loop where fluid recirculates between the two components rather than returning to a tank.

Q2: Where is the 51C110 typically used?

A2: Propel drives on tracked and wheeled construction equipment, winch drives, and various agricultural and forestry machine drivetrains where a wide, continuously variable speed-to-torque range is needed from a compact motor.

Q3: What is the price and delivery time of the 51C110 hydraulic motor?

A3: The specific price and delivery time will be provided based on the specific model you provide.

Q4: How to purchase a 51C110 hydraulic motor?

A4: You can visit https://www.baolilaihydraulic.com/contact/ to contact us and purchase our products through various methods.