In the window coverings industry, the Newton metre (Nm) rating of a motor is often misunderstood and, unfortunately, frequently overstated. When comparing motors, many manufacturers inflate their Nm specifications, which leads to misleading expectations in the market. This has led to a situation where customers have learned to expect certain numbers, making honest ratings appear inadequate at first glance.
To understand torque ratings, let's start with the basics. A Newton (N) is a unit of force, defined by Newton's Second Law: Force = Mass × Acceleration (F = ma). To give a practical example: when you stand on the ground, Earth's gravity pulls you down, and the ground pushes back up with an equal force. For a person weighing 100 kg, this force is about 980 Newtons (100 kg × 9.8 m/s² of gravitational acceleration).
Torque, measured in Newton metres (Nm), is this force multiplied by the distance from the centre of rotation – that's why we use Nm instead of just Newtons. Think of it like this: when you're tightening a bolt with a spanner, you can either push harder (more Newtons) or use a longer spanner (more metres) to achieve the same torque. This is why even a small force applied at the end of a long lever can create significant torque.
In simple terms, a 1 Nm motor should be able to lift a 1 N load at 1 metre away from the centre of the motor shaft. This is, of course, not applicable for window coverings because tube sizes are typically less than 50mm in diameter for interior blinds, and 100mm for exterior blinds.
To use a real-world example for roller blinds, let's calculate the torque required to lift a blind with the following specifications:
The total weight of this blind is 4.35 Kg, which is calculated by adding the total fabric weight (3m * 3m * 0.4 Kg/m² = 3.6 Kg) and total bottom bar weight (3m * 0.25 Kg/m = 0.75 Kg). We disregard the weight of the blind tube as it is evenly distributed and does not require (much) additional torque to turn. If being pedantic, we can also consider the moment of inertia of the system but that is much beyond the scope of this article.
To lift a 4.35 Kg load, the motor must overcome a force of 42.6 N (4.35 Kg * 9.8 m/s²) at a distance of 22.5mm (45mm / 2). The resultant torque required is 42.6 N * 0.0225m = 0.96 Nm. This means that a true 1 Nm motor should be capable of lifting a 3m * 3m blind with the given parameters. A true 2 Nm motor will be able to lift two of these blinds multilinked. However, many manufacturers claim a 2 Nm torque rating while also stating a maximum blind size of 3m * 3m. This size limitation is in direct contradiction to the stated torque capability of the motor.
Rather than focusing solely on Nm ratings when selecting a motor, we encourage our customers to consider real-world performance factors. Does the motor reliably lift your window covering? Does it maintain consistent speed under load? Does it perform reliably over thousands of cycles? These practical considerations are far more valuable than comparing potentially inflated specifications. At Dendo Systems, we believe in providing accurate specifications that you can trust, backed by motors that deliver reliable performance.
