As wear-free brakes, retarder brakes are a central safety element of every truck. Jumo GmbH & Co. KH has been producing special sensors that reliably monitor the temperature in retarder brakes for more than 10 years.
Anyone who regularly drives on German roads will have noticed that truck traffic has increased constantly in recent years. In Germany alone, 3.3 million trucks were registered at the beginning of 2020, making almost 500 million journeys every year. There are also several million trucks from abroad that are on the road every day.
With this development it is clear that the number of accidents in which trucks are involved is also increasing - there were almost 30,000 in 2018, 762 people died. The subject of "technical safety" is therefore becoming increasingly important. The braking system is of particular importance in trucks. Because the braking distance of a fully loaded 40-tonner at 80 kilometers per hour is over 50 percent longer than that of a car.
Trucks usually have four braking systems. A parking, a service, an auxiliary and a permanent brake system. Particular attention is paid to the latter. A retarder system is a generic term for the so-called wear-free brakes. These are intended to relieve the service brake system. Typical representatives are dynamic pressure brakes or retarders.
Retarders work either hydrodynamically (with fluid) or electrodynamically. With retarders, up to 90 percent of all braking processes can be carried out without wear. That is a clear plus in security. Retarders protect the service brakes, which are fully operational in an emergency. At the same time, they reduce the costs for spare parts and maintenance. In addition, the transport performance increases due to higher average speeds.
In order to brake a truck that weighs tons, high braking power is required. In demanding applications, such as adaptive braking on the motorway or long downhill stretches, classic friction brakes can reach temperatures of up to 1,000 degrees. As a result, their braking effect drops rapidly.
In the area of hydrodynamic retarders in particular, temperature measurement is essential for optimal operation. These work with oil or cooling water, which is fed into a converter housing if necessary. The converter housing consists of two opposing paddle wheels, a rotor that is connected to the vehicle's drive train, and a stationary stator. The rotor accelerates the supplied medium; centrifugal force pushes it outwards. Due to the shape of the rotor blades, the water or oil is directed into the stator, which thereby brakes the rotor and subsequently also the cardan shaft.
Simply put: The hydrodynamic retarder is a type of pump that pumps liquid in a circuit outlet. The braking effect is created by the work resistance of the pump, which can be regulated. Both extremely precise and quick temperature measurements are required. In the worst case, incorrect measurements of the oil or water temperature in the retarder can even lead to increased wear and tear on the entire brake system.
Jumo Vibrotemp temperature sensors are used to monitor the temperature of the medium in the retarder in order to indirectly guarantee the optimum operating temperature of the medium and thus optimum braking performance. The screw-in resistance thermometers of the Vibrotemp series enable reliable temperature measurements even under pressure in commercial vehicles, construction machinery, agricultural machinery, engines, compressors and in rail technology. Thanks to the vibration-proof construction, excellent long-term stability is achieved even under rough operating conditions.
Depending on the area of application, you can choose between different plug connections and protective fitting materials. When plugged in, protection class IP67 / IP69 according to DIN EN 60529 or IP69K according to DIN 40050 is fulfilled. Due to the special geometry of the protective fitting, a particularly fast response time can be achieved (t0.5 = 1.2 s, t0.9 = 6 s). A Pt100 temperature sensor according to DIN EN 60751, class B in two-wire circuit is used as a measuring insert as standard. Versions with Pt500 or Pt1000 as well as PTC or NTC temperature sensors are also possible.
Depending on the version, the Vibrotemp product family can cover measuring ranges from -50 ° C to + 270 ° C, and even 300 ° C for a short time. Frequencies from 10 to 3000 Hz, acceleration up to 30 g or shock shocks up to 100 g / 5 ms cause no problems for this compact temperature sensor.
In the application example described, Vibrotemp sensors reliably measure oil temperatures of up to 180 degrees Celsius and cooling water temperatures of up to 150 degrees Celsius and are used in retarder brakes by numerous well-known truck manufacturers.
By Lukas Bolz, Jumo Product Manager