Hydraulic Drive Crawler Undercarriage Motor Selection: Displacement, Pressure and Control

Quick answer: Motor displacement is derived from the machine weight, design slope and ground condition, then converted to sprocket torque and motor displacement through the drive efficiency. A two-speed motor with a 2:1 to 3.5:1 ratio is standard above 5 tons, and the oil cooling must be sized for the calculated heat rejection.

The hydraulic drive motor is the single most consequential component choice on a custom undercarriage, and it is frequently specified by analogy — "the same motor as the previous machine" — rather than by calculation. When the motor is right, the machine has precise control, adequate speed under load, and no thermal problems. When it is wrong, the symptoms appear months later as slow travel under load and an overheating drive that has to be derated.

This is how MAGEON selects the drive for a crawler undercarriage, and what we ask a customer to confirm when they already have a motor in mind.

The calculation: what torque does the machine need?

Start with the tractive force required at the shoe-to-ground interface:

F = W × g × sin(θ) + W × g × C_rr

where W is the machine weight, θ the maximum design slope, and C_rr the rolling resistance coefficient of the track on the ground. Typical C_rr values: 0.05 on firm compacted soil for a steel track, 0.07 on loose ground, 0.04 on rock for a steel track, and 0.10 for a rubber track on soft ground.

Then the torque at the drive sprocket:

T = (F × r_sprocket) ÷ η_drive

with r_sprocket the sprocket pitch radius and η_drive the combined efficiency of the motor, gearbox, and final drive, typically 0.85–0.90.

Finally the motor torque from its displacement and system pressure:

T = (Δp × V) ÷ 2π

where Δp is the pressure differential across the motor and V is its displacement per revolution. With Δp in bar and V in cm³/rev, torque comes out in N·m.

The worked example

Machine: 25-ton crawler undercarriage, design slope 30°, firm compacted soil, steel track, 350 mm sprocket pitch diameter.

• W = 25,000 kg; W×g = 245,250 N
• Slope component: 245,250 × sin 30° = 122,625 N
• Rolling resistance: 245,250 × 0.05 = 12,263 N
• Required tractive force F = 134,888 N
• Torque at sprocket: 134,888 × 0.175 ÷ 0.87 = 27,130 N·m
• Selecting a 250 cm³/rev motor at 250 bar: T = (250 × 10⁵ × 250×10⁻⁶) ÷ 2π = 9,947 N·m — insufficient
• A 630 cm³/rev motor at 250 bar gives 25,070 N·m; a 700 cm³/rev motor gives 27,850 N·m, which clears the requirement with margin

This calculation is why motor selection cannot be copied between machines. A 25-ton machine and a 30-ton machine differ not only in weight but in the ratio of slope load to rolling resistance, and the correct displacement can differ by 30%.

Two-speed: the standard solution

Almost every crawler undercarriage above 5 tons uses a two-speed motor — a swashplate motor with two displacement settings, typically a high-displacement low-speed range for travel and work, and a low-displacement high-speed range for light travel and repositioning.

The reason is that a single-displacement motor must be sized for the worst case. Used at that displacement for fine positioning, it moves slowly and wastes fuel and energy through throttling losses. The two-speed arrangement gives roughly a 3:1 speed range with a modest cost increase, and it is a large contributor to the productivity difference between a well-specified machine and a poorly specified one.

MAGEON typically uses a 2:1 to 3.5:1 displacement ratio, with the high-speed range selected automatically by the machine's own control logic above a travel speed threshold.

System pressure and its consequences

Raising the system pressure from 200 to 250 bar reduces the required motor displacement by 20% for the same torque. That sounds like a free win, and it partly is — but the consequences run the other way:

• Component cost rises across the pump, motor, hoses, and valves
• Heat generation rises because the pressure differential across the orifice is greater, and this heat must go somewhere
• System efficiency falls below about 200 bar for a high-flow system, because losses scale roughly with the square of the pressure drop
• Component life falls in fatigue terms, since fatigue life scales with the inverse of the stress range to a high power

Our practice is to specify the lowest system pressure that reliably meets the torque requirement with margin at the machine's maximum ambient temperature, and to accept the slightly larger motor in preference to running components harder than necessary.

Motor mounting, and why the position matters

The motor is normally mounted directly in the sprocket housing, with the pinion engaging the sprocket teeth. Two consequences follow:

Radial load on the motor shaft. Chain tension applies a radial force to the pinion that the motor bearings must carry. This is why a crawler undercarriage motor is specified as a heavy-duty, large-diameter, low-speed unit rather than a standard mobile hydraulic motor, and why the chain tension and the motor's radial load rating have to be considered together.

Mounting position and the centre of gravity. The drive motor is the heaviest single item on the chassis. Its fore-aft position directly affects the machine's weight distribution and therefore the front and rear roller load split. On a custom undercarriage this is a design variable, and moving the drive rearward is a common technique for machines that spend time digging far forward.

Control valve architecture

Three architectures cover almost all applications:

Note that "closed centre" is often sold as a productivity feature; its real value is energy efficiency and better heat behaviour. If the customer's machine alternates between travel and work every few minutes — a skid steer, a mini excavator — the regenerative benefit of closed centre is substantial.

Thermal management, the item most often overlooked

Drive motors reject heat into the hydraulic oil, which rejects it into the tank, which must be sized to dissipate it. The failure mode is not immediate. It is a machine that runs fine in the morning, runs hotter through the afternoon, and eventually derates itself on a hot day in a project with a schedule.

MAGEON specifies the tank volume from the calculated heat rejection, verifies the oil routing back to the motor, and confirms the cooling approach —natural convection, an oil cooler, or a water-cooled exchanger — as part of the chassis package. We also specify the oil type and the permissible operating temperature range, because an oil chosen for cost can be the limiting factor on a machine working in a hot climate.

What to confirm when specifying a motor

1. Is the displacement derived from a torque calculation at the machine's maximum design slope, or carried over from a previous model?
2. Does the motor have a two-speed range, and what is the ratio?
3. Is the motor specified for the radial load from the chain tension, not only for torque?
4. What is the calculated heat rejection, and how is it dissipated?
5. What is the system's maximum continuous pressure, and is every component rated for it with margin?
6. Is the motor's mounting position compatible with the machine's intended weight distribution?

Conclusion

Hydraulic drive selection for a custom undercarriage is a closed calculation: weight and slope determine force, force and sprocket diameter determine torque, torque and pressure determine displacement, and the thermal duty determines the cooling. Every one of those steps has a defensible engineering answer, and a customer who specifies the motor by calculation rather than by precedent gets a machine that is more controllable, more efficient, and easier to support. MAGEON will run this calculation against your inputs and show the working.

MAGEON product lines and how they map to this topic

The engineering principles in this article apply across the full MAGEON product range. Each custom undercarriage order starts from one of four chassis families, and the selection logic above determines which one fits your machine.

Whether you are specifying a custom undercarriage from first principles, comparing a customized steel track chassis against a customized rubber track chassis, or reviewing the service life of a customized steel track undercarriage in abrasive service, MAGEON can size, engineer and support the build. Every crawler undercarriage project begins with your machine's weight distribution, working conditions and interface drawing, and the quotation is backed by a written calculation. Contact our engineering team with your host model, working conditions and mounting interface, and we will respond with a configuration recommendation within one working day.