1. The Field Problem: A Backstop That Cooks Its Own Grease

A fertilizer plant reported that the backstop on a belt conveyor drive was running at 95–100 °C after two hours of freewheeling, with grease weeping from the seal and a burnt smell. The bearing had never carried load — it only freewheeled whenever the conveyor ran forward. The maintenance team assumed the unit was "too small" and proposed a larger one. In fact the unit was oversized: a backstop designed to hold 6 kN·m was installed on a drive that needed 1.2 kN·m, and at 1,500 rpm the continuous drag of the oversized sprag set was dissipating several hundred watts inside a housing with almost no way to shed it.

Drag torque is the only continuous heat source in a backstop, and it is almost never specified in a quotation. This article sets out where it comes from, how to calculate what it does to temperature, and how to measure it on site.

2. Mechanism: What Creates Drag, and What Removes It

2.1 Spring preload and sprag rub

Sprags are energised against both races by a cage spring so that they can engage immediately in either direction of reversal. While freewheeling, each sprag is dragged along the race with a small normal force, and the sum over 20–40 sprags produces a resisting torque. Below the centrifugal lift-off speed this rub is continuous.

2.2 Centrifugal lift-off

Above a characteristic speed the sprag mass throws them clear of the inner race and drag torque falls sharply. The lift-off speed n_c is a designed feature of each series and must be checked against the actual maximum freewheeling speed. Two consequences follow: running below lift-off means continuous rub and heat; running far above lift-off means the sprags must travel further and engagement response slows, which is why lift-off speed and engagement time are quoted together.

2.3 Grease churn and fill quantity

Over-greasing is a classic self-inflicted heat source. A housing filled to 100 percent churns its grease at high speed; the same housing at 30–50 percent fill runs 20–30 K cooler for identical drag from the sprags. This is one of the fastest fixes available in the field.

2.4 Seal friction

Radial lip seals add a nearly constant torque term. It grows with shaft speed and with seal hardness — a seal that has dried out after a long shutdown can add more drag than the sprags themselves.

3. Worked Example: From Drag Torque to Housing Temperature

Case: sprag backstop, measured drag torque 2.4 N·m at 1,500 rpm, housing external area 0.15 m², natural convection coefficient about 12 W/(m²·K) for an unstirred indoor environment, ambient 30 °C.

The arithmetic is deliberately blunt: a few newton-metres of drag at conveyor speed is hundreds of watts, and a sealed grease housing can shed only tens of watts by convection. This is why drag torque and cooling method must be selected together, and why oil-lubricated gearbox-mounted units tolerate high speed far better than sealed grease units.

A practical rule used in selection: at maximum freewheeling speed, keep the calculated heat generation below roughly 5–8 W per 0.01 m² of exposed housing area for sealed units, or arrange positive oil flow for anything above that.

4. Engineering Solutions

• Right-size, do not up-size. Choose the smallest holding torque that satisfies the reverse-torque calculation with its margins. Excess capacity is paid for in drag and heat every hour of operation.
• Reduce grease fill. Specify 30–50 percent housing fill for high-speed freewheeling duty; verify with the manufacturer's recommended quantity, not with "fill until it comes out".
• Use lift-off to your advantage. Select a series whose lift-off speed sits below maximum running speed so that steady-state drag is low, then confirm engagement response is still acceptable for the duty.
• Provide cooling. Gearbox-mounted oil-lubricated units should receive the gearbox oil flow; standalone units may need an oil circuit or a finned/ventilated housing.
• Trend temperature. A rise of 10–15 K above commissioning baseline is a reliable early warning of seal drag, grease degradation or bearing distress.

5. Series Selection and Verification

Ask the supplier for: drag torque at the actual maximum freewheeling speed (not a single figure at unspecified speed), centrifugal lift-off speed with the associated engagement time, recommended lubricant quantity and fill level, permissible continuous housing temperature and seal limit, and the cooling method assumed in the rating. If drag torque is not on the datasheet, request it in writing before the order.

6. FAQ

Q: The backstop never carries load. Why does it get hot?
A: Because sprags are spring-loaded against both races and rub continuously below lift-off speed. Drag torque × speed is real power and it has nowhere to go in a sealed housing.

Q: Will a bigger backstop run cooler?
A: Usually the opposite. Larger units have more sprags and higher drag; oversizing is a common cause of overheating on freewheeling drives.

Q: How do I measure drag torque on site?
A: Run a coast-down test: drive the shaft to a known speed with the load disconnected, record the deceleration, and compute T = J·dω/dt. Alternatively measure the no-load driving torque with a torque transducer during a slow rotation.

Q: Is high temperature only a lubrication problem?
A: No. It is a thermal-balance problem. Changing grease buys time; correcting drag, fill level or cooling solves it.