A timing belt that jumped teeth after 2 weeks. The drive was a 24V DC gearmotor, 50 RPM output, driving a conveyor at 0.3 m/s. The belt was HTD 5M, 15 mm wide. The customer re-tensioned it. It jumped again after another week. The issue: the initial tension was too low for the belt width and span. This is about timing belt initial tension.

The initial tension formula

The belt must be pre-tensioned so it doesn’t slip or jump teeth under load. The recommended initial tension for a 5M belt:

F_0 = 0.005 × W × (C / 100)

Where W is the belt width (mm) and C is the center distance (mm). For a 15 mm wide belt at 500 mm center distance: F_0 = 0.005 × 15 × 5 = 0.375 kgf = 3.7 N. That’s very low. But the rule of thumb for a 15 mm HTD belt is actually 3-5 kgf (30-50 N). The formula above is for light loads.

The real tension depends on the torque. The tight-side tension:

F_tight = T / (r_pulley)

For a 50 RPM gearmotor at 10 N·m output torque on a 50 mm pitch pulley (r=25 mm): F_tight = 10,000 / 25 = 400 N. The slack-side tension is about 20% of tight-side = 80 N. The average tension is 240 N. The initial tension must be about 50% of the average = 120 N. That’s 12 kgf. The customer had it at 3 kgf. Way too low.

What I changed

1. Increased initial tension to 10 kgf. I adjusted the center distance to achieve 10 kgf (measured with a tension gauge). The belt no longer jumped teeth. At 10 kgf initial tension, the tight-side tension under load is 120 + 400/2 = 320 N. The slack side is 120 – 200 = negative… wait, that doesn’t work. The initial tension should be about 1/3 of the tight-side tension. At 120 N (12 kgf), the belt has enough wrap to transmit 400 N tight-side without jumping.

2. Added a tensioner spring. For conveyor applications where the belt stretches over time, I add a spring-loaded tensioner. The spring maintains the initial tension as the belt wears. Without it, the belt loosens after a month and jumps. The tensioner costs $20 and prevents a $200 belt replacement.

3. Checked pulley alignment. The pulleys were misaligned by 2 mm. A misaligned belt runs on one flange, wearing the edge and reducing tension. I aligned the pulleys with a straightedge. The belt ran centered. The edge wear stopped.

The tension measurement

I measure belt tension with a frequency method: pluck the belt like a guitar string, measure the frequency with an app, and calculate tension from the formula. For a 15 mm HTD belt at 500 mm span: the target frequency is about 80 Hz. If it rings at 40 Hz, the tension is half. If at 120 Hz, it’s 2x. The frequency method is more accurate than guessing by feel.

Too much tension

Over-tensioning destroys bearings. A 15 mm belt at 20 kgf (200 N) pushes on the pulley shaft with 200 N force. The bearing life drops by (1/2)^3 = 1/8. I never exceed 1.5x the recommended tension. The sweet spot is: tight enough not to jump, loose enough not to over-load bearings.

The tension I set: about 1/3 of tight-side tension, verified by frequency method. The jumping belt wasn’t defective — it was tensioned at 3 kgf when it needed 10 kgf. Add a spring tensioner for conveyors. Align the pulleys. Tension by frequency, not by feel.