The machine starts, oil flows, but the conveyor barely turns and the sweeper brush stops at the slightest resistance? A mechanic's first thought is usually: "The pump isn't providing pressure, time to buy a new one!". But before you embark on a costly replacement of the system's heart, take a look at its "legs" – the hydraulic motor.
BRhydraulic experts have taken a closer look at the most popular orbital (gerotor) motors on the market. Find out how to correctly read parameters, avoid selection errors, and why a "bigger motor" does not necessarily mean better machine performance.
1. The Magic of the Orbital Motor (Why is it so popular?)
Orbital motors (often found under designations such as BMR, BMP, SMS, or Danfoss) are absolute workhorses in agriculture, forestry, and industry. Their design is based on an internal gear set (the so-called gerotor). Oil pumped by the pump moves the internal gear, which uniquely "rolls" along the external ring.
What are the main advantages of this solution? Primarily, enormous torque at relatively low speeds and very compact dimensions. This makes it an ideal choice for driving winches, drills, wrappers, spreaders, and conveyor feeds.
2. Motor Displacement: Rotational Speed is Pure Mathematics
The main parameter of any motor is its displacement (working volume), expressed in cubic centimeters per revolution (cm³/rev). This parameter tells us how much oil the motor needs for its shaft to complete one full revolution (360 degrees).
The rule is absolute: The rotational speed of the motor depends on the oil flow from the pump (liters per minute) and the motor's displacement itself.
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If you have a constant flow pump (e.g., 40 l/min):
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A small motor with a displacement of 50 cm³/rev will rotate very quickly (below 800 rpm).
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A large motor with a displacement of 400 cm³/rev will rotate very slowly (only about 100 rpm) when supplied by the same pump!
3. Torque: Where Does the Power Come From?
If speed depends on oil flow, then what determines the force with which the motor turns the shaft (i.e., torque given in Nm)? The answer is working pressure.
The larger the motor's displacement (i.e., physically larger gears inside), the greater the torque it will generate at the same pressure. A larger area on which the oil exerts pressure translates into a more powerful lever and greater force. That's why powerful forestry drills use large motors that rotate slowly but with immense, unrelenting force.
4. Common Pitfalls – How Not to Damage the System?
Customers often look for the "biggest motor," assuming that it will blindly provide the best results. Let's see how such experiments end in practice:
| Your selection error | Consequence in the machine system | How to solve this problem? |
| Too large displacement (e.g., 400 cm³/rev instead of 100) | The motor has immense power but rotates dramatically slowly. The machine loses efficiency, and work "stalls." | Select a smaller motor (with lower displacement) or install a much larger pump to increase flow rate. |
| Too small displacement (e.g., 50 cm³/rev instead of 200) | The motor spins wildly but stops under the slightest load (lack of torque). | Select a larger motor or use a mechanical gearbox (reducer), converting speed into force. |
| Lack of an overload valve | If the machine suddenly jams, the motor shaft stops. Pressure drastically increases and ruptures the seals. | Always use a cross-over (shock) valve mounted directly on the motor connections. |
The ideal selection of a hydraulic motor is always a compromise between the desired rotational speed and the required force. Before purchasing a new component from a catalog, thoroughly check your pump's parameters (how many liters per minute it provides and what nominal pressure it can generate).
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