MF21-031A-45 Sauer Danfoss Axial Piston Motor 51.6cc

Product Overview

The MF21-031A-45 is a mid-displacement swash plate axial piston fixed motor from the Sauer Danfoss (now Danfoss Power Solutions) Series 20, MF21 frame size. The motor employs a swash plate construction with preset displacement, specifically designed for closed-circuit hydrostatic transmissions. Output speed is proportional to the flow rate of the input fluid, and output torque is proportional to the differential between high and low pressure sides of the fluid circuit. The direction of motor shaft rotation depends upon which port the fluid enters the motor. The Series 20 is renowned for its mature, reliable design and ease of handling.

The MF21 offers a maximum displacement of 51.6 cm³/rev (3.15 in³/rev), rated working pressure of 21 MPa (210 bar), maximum pressure of 35 MPa (350 bar), maximum speed of 3500 rpm, nominal speed of 1500 rpm, maximum flow of 180.6 L/min, and weight of approximately 34 kg. This model is primarily designed for concrete mixer truck drum drive systems, compatible with mixer drum capacities of 6–10 m³.

The Series 20 covers eight mainstream models — MF20, MF21, MF22, MF23, MF24, MF25, MF26, and MF27 — with maximum displacements ranging from 33.3 cm³/rev to 333.7 cm³/rev and maximum flows from 119.5 L/min to 557.2 L/min, forming a complete power matching system. These motors can be paired with Sauer PV series pumps (PV20–PV27) to form complete closed-circuit hydraulic systems.

The model code decodes as: “MF21” = MF series 21 frame size, “031” = control and configuration code, “A” = design revision, “45” = mounting flange and shaft specification.

The motor offers complete interchangeability: mounting dimensions, port positions, and standardized interfaces are identical to the original Sauer Danfoss specification, enabling seamless pairing with Sauer PV series pumps in closed hydraulic systems. Every unit undergoes strict factory pressure testing, sealing testing, and performance calibration before shipment.

Technical Specifications 

MF21-031A-45 (Primary Model)

• Model: MF21-031A-45

• Brand Series: Sauer Danfoss / Danfoss Series 20 / MF21

• Motor Type: Swash plate axial piston fixed motor, closed-circuit dedicated

• Maximum Displacement: 51.6 cm³/rev (3.15 in³/rev)

• Rated Working Pressure: 21 MPa (210 bar)

• Maximum Pressure: 35 MPa (350 bar)

• Nominal Speed: 1500 rpm

• Maximum Speed: 3500 rpm

• Maximum Flow: 180.6 L/min

• Charge Pump Displacement: 12.3 cm³/rev

• Maximum Control Pressure: 3.5 MPa (35 bar)

• Charge Pressure: 0.8–2.0 MPa (8–20 bar)

• Case Pressure: 0.25 MPa continuous / 0.5 MPa intermittent

• Maximum Swash Plate Angle: ±18°

• Mounting: Flange mount

• Rotation: Bi-directional (depends on port entry direction)

• Shaft Type: Full-length shaft with high-efficiency tapered roller bearings

• Weight: Approximately 34 kg

• Operating Temperature Range: -40°C to +80°C (maximum tank oil temperature)

• Fluid Cleanliness Requirement: 10 µm

Series 20 Other Displacement Sizes (Series Reference)

• MF20: Max displacement 33.3 cm³/rev, max flow 119.5 L/min, max speed 3590 rpm, weight 30 kg

• MF21: Max displacement 51.6 cm³/rev, max flow 159.9–180.6 L/min, max speed 3500–3100 rpm, weight 34 kg

• MF22: Max displacement 69.8 cm³/rev, max flow 196.1 L/min, max speed 2810 rpm, weight 40 kg

• MF23: Max displacement 89.0 cm³/rev, max flow 230.5 L/min, max speed 2590 rpm, weight 47 kg

• MF24: Max displacement 118.7 cm³/rev, max flow 278.9 L/min, max speed 2350 rpm

• MF25: Max displacement 165.8 cm³/rev, max flow 348.1 L/min, max speed 2100 rpm

• MF26: Max displacement 227.3 cm³/rev, max flow 429.6 L/min, max speed 1890 rpm

• MF27: Max displacement 333.7 cm³/rev, max flow 557.2 L/min, max speed 1670 rpm

Note: Series parameters are compiled from publicly available technical data on hydpump.com and xeriwell.com.

PV22 / PV23 Matching Pump Parameters (Series Reference)

• PV22: Displacement 69.8 cm³/rev, rated pressure 210 bar, max pressure 350 bar, max speed 3200 rpm, weight 60 kg

• PV23: Displacement 89.0 cm³/rev, rated pressure 210 bar, max pressure 350 bar, max speed 2900 rpm, weight 75 kg

• PV22/PV23 pumps with MF22/MF23 motors form closed hydraulic systems widely used in concrete mixer trucks, underground LHDs, and road rollers

Key Technical Advantages

① Swash Plate Construction with Hydro-Mechanical Servo Control — Precise Displacement Locking and Stepless Speed Regulation

The MF21-031A-45 employs a swash plate construction equipped with a professional hydro-mechanical servo displacement control system. This system precisely locks the swash plate angle and displacement parameters during operation, achieving stepless speed regulation and flexible forward and reverse rotation. When the control handle is reset, the swash plate automatically returns to the null position to cut off hydraulic flow, effectively avoiding power waste and operation jitter, significantly improving the smoothness and overall working efficiency of mechanical operation. For concrete mixer trucks and other applications requiring precise drum speed control, this control characteristic ensures speed stability during mixing and discharging.

② Full-Length Shaft with High-Efficiency Tapered Roller Bearings — High Load Capacity and Long Service Life

The MF21 features a full-length integral shaft with high-efficiency tapered roller bearing arrangement. Compared to ordinary hydraulic motors, this design dramatically improves the bearing capacity for external radial forces, effectively avoiding shaft deformation, jitter, and premature wear caused by long-term heavy-load operation. During concrete mixer truck travel, impact loads from road bumps are uniformly distributed through the tapered roller bearings, extending motor overhaul intervals. The full-length shaft design also supports additional motor mounting at the shaft end, expanding application flexibility.

③ High-Strength Alloy Materials with Precision Sealing Components — Adaptable to Demanding Conditions

The entire unit is manufactured from high-strength alloy materials with imported precision sealing components, resisting high temperature, high pressure, dust, and humid harsh working environments. Internal leakage rate and failure rate are extremely low, greatly reducing equipment downtime and later maintenance costs. For construction sites, mining, and other dust-intensive operating environments, this material and sealing configuration ensures stable performance output during long-term use.

④ Modular Design for Easy Maintenance — Reducing Downtime Costs

Series 20 motors feature a modular structural design that simplifies manufacturing of different model variants while limiting the number of different components involved. The axial piston units are designed for easy servicing; complete dismantling and reassembly can be carried out with standard hand tools, and all components or sub-assemblies are replaceable. For fleet operators of concrete mixer trucks, this means single-motor maintenance can be completed on-site quickly without complete replacement, significantly reducing maintenance costs and downtime.

⑤ 100% Interchange Compatibility with Standardized Interfaces — Drop-In Replacement Without Modification

The MF21-031A-45’s standardized overall dimensions, mounting interfaces, and port positions are identical to the original Sauer Danfoss specification, achieving 100% interchangeable assembly. For old equipment maintenance and parts replacement, no pipeline modification or structural debugging is required to complete quick replacement and put into operation. The series forms a perfect matching closed hydraulic system with Sauer PV series pumps, providing convenience for overall maintenance of mixer truck hydraulic systems.

Application Areas

• Concrete Mixer Trucks (Primary Application): Mixer drum drive systems, compatible with 6–10 m³ drum capacities, compatible with Sany, Zoomlion, XCMG, and other mainstream mixer truck brands

• Underground LHDs: Travel drives and working implement drives

• Road Rollers: Vibratory drum drives

• Combine Harvesters: Travel drives and working component drives

• Construction Machinery: Mini excavators, loader auxiliary drives

• Agricultural Machinery: Tractor hydraulic transmission systems

• Others: Mobile machinery and industrial equipment requiring closed-circuit hydrostatic transmission

Expert Maintenance Tips

Pre-Installation Inspection: Verify flatness and cleanliness of the mounting flange mating surface; check thread condition of mounting bolt holes. The full-length shaft-to-coupling or drive flange fit must conform to standards, avoiding interference fits that could damage the shaft. Confirm tapered roller bearing lubrication condition before installation. Use bolts of strength class 8.8 or higher, tightened in diagonal sequence to specified torque.

First Commissioning: The motor casing must be filled with clean hydraulic oil before initial start-up. Fill through the highest casing port until oil overflows from the level port. Dry starting causes metal-to-metal contact between piston-to-bore surfaces and the valve plate without lubrication, resulting in irreversible damage. Closed-circuit systems should complete charge pump circuit bleeding before startup, confirming charge pressure within the 0.8–2.0 MPa range.

Fluid Cleanliness and Temperature Control: Maintain system oil cleanliness at 10 µm or better. The swash plate piston motor’s valve plate and piston pairs are sensitive to particulate contamination; excessive contamination accelerates wear and causes volumetric efficiency degradation. Viscosity range: 1000 mm²/s at starting, 12–600 mm²/s operating, 25–35 mm²/s optimum. Maximum tank oil temperature not exceeding 80°C. VG46 or VG68 viscosity grade hydraulic oil recommended. Inspect return filter elements regularly and replace per the machine manufacturer’s recommended interval.

Scheduled Inspection: Check casing drain flow and output shaft seal condition every 500 operating hours. Under normal conditions, drain flow should remain stable; a sustained increase indicates accelerated wear of piston pairs or valve plate surfaces, and preventive maintenance should be scheduled. Also inspect servo controller response linearity; if displacement regulation exhibits lag or non-linearity, inspect servo spool and charge pump operation. For mixer truck applications, pay particular attention to the effects of drum load variations on the motor, periodically checking mounting bolt tightness.

Long-Term Storage Protection: If the motor will be idle for more than 3 months, fill the casing with rust-preventive hydraulic oil and manually rotate the shaft several turns monthly to prevent corrosion or adhesion on piston pairs and valve plate surfaces. Perform a no-load run-in before returning to service.