Do brakes, bearings and friction need to meet the temperature class?
Abstract
Yes. Frictional heat from brakes, bearings, seals and couplings counts as a hot surface. Under ISO 80079-36 its maximum temperature must stay within the marked class, assessed in normal operation for Gc, also in expected malfunctions for Gb, and in rare malfunctions for Ga.
Yes. A brake disc, a running bearing, a mechanical seal or a slipping coupling heats surfaces by friction, and ISO 80079-36 treats such surfaces like any other hot surface: their maximum temperature, including the safety margins of the standard, must stay within the temperature class marked on the equipment. Which situations are included depends on the equipment protection level (EPL): normal operation for Gc, expected malfunctions for Gb, and rare malfunctions as well for Ga.
Short answer
- Frictional heat is an equipment-related ignition source. ISO 80079-36 names both cases: a brake that is hot in normal functioning and a bearing that overheats when it fails.
- The temperature class (or marked temperature) of
Ex hequipment covers every surface that the explosive atmosphere can reach, including those heated by friction. - Gc: normal operation. Gb: normal operation and expected malfunctions, such as wear or exceeded lubricant life. Ga: also rare malfunctions, such as bearing failure, and two expected malfunctions together.
- Where a surface could exceed the limit in a malfunction the EPL must cover, the manufacturer adds protection, for example temperature or vibration monitoring with automatic switch-off (type of protection "b") or constructional safety (type "c").
- Friction also produces sparks; that is assessed separately from the temperature.
What the standard requires
ISO 80079-36, the basic standard for non-electrical equipment such as pumps, gearboxes, couplings, fans and brakes, requires an ignition hazard assessment of the complete equipment. For hot surfaces it requires the manufacturer to establish the highest surface temperature at the worst-case working conditions and the maximum ambient temperature, then compare it with the marking:
| EPL | Conditions included when determining the surface temperature | Margin to the marked class (type-tested equipment) |
|---|---|---|
| Gc | Normal operation | 5 K below the class limit for T3 to T6, 10 K for T1 and T2 |
| Gb | Normal operation and expected malfunctions | 5 K or 10 K, as for Gc |
| Ga | Expected and rare malfunctions | Highest temperature at most 80 % of the marked value or of the AIT of a named gas |
The larger margin for Ga is explained in the standard itself: rare mechanical malfunctions are hard to reproduce in a temperature test.
Where the surface temperature depends mainly on the process, as in a pump handling a hot liquid, the equipment is marked with a range, such as T6...T4, and the instructions tell the user which class applies at which process temperature.
Bearings, brakes and other friction sources
Bearings. ISO 80079-36 uses bearing overheating as its example of ignition resulting from a failure in the machine. In practice a bearing overheats after loss of lubricant, overload or contamination. For Gb equipment, the standard's examples of expected malfunctions the design must handle include mechanical wear, exceeded service life of the lubrication and a seal running without lubrication, with level monitoring and switch-off as an adequate measure. For Ga, ISO 80079-36 gives a bearing failure as a rare malfunction example and names continuous bearing monitoring with switch-off as a suitable measure.
Brakes and clutches. These convert energy into heat as part of their function, so the hot friction surface exists in normal operation. The design has to show that the surface stays within the class at the worst duty, for example the most frequent stops or the longest slip time the manufacturer allows, or the brake must be enclosed or placed so that the explosive atmosphere cannot reach it.
Seals and couplings. Mechanical seals can run dry and fluid couplings can overheat on overload. ISO 80079-36 mentions a low melting point fusible plug in a fluid coupling as an example of a temperature-limiting device that must be taken into account.
Low speeds. A note in the standard says that a relative contact speed of 1 m/s is often used as the limit below which friction is not considered able to ignite, with exceptions: dust trapped in gaps (for example in bearings and seals), extremely sensitive dusts such as sulphur, hydrogen and ethylene under high contact load, and other sensitive mixtures such as acetylene, carbon disulphide, carbon monoxide and ethylene oxide.
Example: A gearbox marked Ex h IIB T4 Gb is installed in Zone 1. Its instructions require oil level monitoring. If the oil runs low, the bearings and gears heat up; this is an expected malfunction for Gb. The temperature class remains valid only if the level switch is wired to stop the drive, as specified.
What the user must do
- Keep the conditions the temperature class depends on: lubrication intervals, oil level, alignment, maximum speed, braking duty.
- Connect and maintain any monitoring device the manufacturer specifies; it is part of the protection.
- Check process heat: a pump marked with a class range is only as cool as the liquid allows.
- In dust zones, keep layers off hot bearing housings; dust layers insulate and raise the temperature.
Common mistakes
- Assuming mechanical parts need no temperature class. Non-electrical equipment with its own ignition sources needs an assessment and marking like electrical equipment.
- Judging only by the motor nameplate. The driven machine, coupling and brake have their own surface temperatures.
- Disabling bearing temperature switches after nuisance trips. If the certificate relies on them, the class no longer holds.
Related on excodex
- Hot surfaces in hazardous areas: luminaires, motors, heat tracing and process heat
- Non-electrical equipment in hazardous areas: Ex h, constructional safety c, control of ignition sources b and liquid immersion k
- Do pumps, gearboxes and fans need ATEX certification?
- Is the temperature class based on normal operation or also on faults?
- Standards spaces: ISO 80079-36, ISO 80079-37
Standards and further reading
- ISO 80079-36:2016, non-electrical equipment for explosive atmospheres: basic method and requirements (hot surfaces, friction, EPL-dependent determination of surface temperature, safety margins). Public text: law.resource.org; catalogue entry: standards.iteh.ai
- ISO 80079-37:2016, non-electrical type of protection constructional safety "c", control of ignition source "b", liquid immersion "k".
- EN 1127-1:2019, explosion prevention and protection: basic concepts (ignition sources including hot surfaces and mechanical sparks).
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.