Off Highway Solutions


The mechanical, thermal and electromagnetic requirements for the electric powertrain in vehicles are complex. Thomas uses its know-how to find efficient and high-performance system solutions in this area, which is used, e.g. in the automatic transmissions or to control the cooling circuit for the batteries.

Do you have any questions about our products? We have compiled the most frequently asked questions and their answers for you here. Should you not be able to find an answer to your inquiry here, we will be happy to advise you personally. You can find the contact person under points of contact.

Application and Conditions

The hydraulic valves of Thomas are tested and validated in development and in series production with HLPD32 or HLPD46 oil from Texaco Rando. The oil temperature in operation is at 50°C (viscosity at 50°C between 22cSt - 28cSt).

The hydraulic properties of the valves are mainly tested under these boundary conditions and are specified in the specifications. In principle, other hydraulic oils can be used. The properties of special oils can slightly influence the technical characteristics of the valve. Their use should therefore be consulted in individual cases.

On general knowledge more than 50% of all problems in a hydraulic system are the result of the pollution in the used medium. A distinction is made between three types of contamination: solid, gaseous and liquid contaminants. Referring to the pollution caused by solid particles in the oil, the oil cleanliness required for the use of Thomas valves is according to specifications across all products at least 20/18/15 (ISO 4406) or better.

The most temperature depending component in a hydraulic valve is the installed coil. Due to its compact shape and high power density, the highest temperature can be achieved during intensive operation and high ambient temperatures. Different temperature influences act upon a valve. When defining limits of permissible temperatures these individual influences should be considered separately:

1.    The outer / air temperature (air temperature possibly with various external sources of influence such as: exhaust line, etc.) permissible air temperature: -30°C to + 80°C 
2.    The block temperature (the block / cavity in which the valve is installed has a certain operating temperature) permissible block temperature: max. 80°C at the valve flange surface at 100% duty cycle with Imax 
3.    The oil temperature (the hydraulic oil flows through the valve or into the armature space) permissible oil temperature: -30°C to + 105°C

In operation, the valve reaches under the influence of these three temperature sources at maximum height (80°C, 80°C, 105°C) and at maximum energizing a steady-state temperature of 175°C.

The coils of the Thomas valves are designed in a way, that even under the most demanding conditions (continuous operation of the valve and highest temperatures defined above) damage to the coil is almost impossible. But in extreme cases the rising temperatures may limit the technical characteristics of the valve. Due to the rising temperature the coil resistance increases, so that a higher driving voltage is required. At very high temperatures in the coil (> 180 ° C), the maximum current can often no longer be generated by the coil (especially at relatively low supply voltages), and the valve can no longer build the maximum working pressure.

With an increasing temperature of the coil the coil resistance increases as well (see also the question: usability up to which temperature (min voltage)). This way, a higher voltage is needed in order to apply the maximum current. The customer is urgently requested to pay attention which supply voltage may occur in the worst-case and which maximum operating temperatures can prevail at the same time, in order to ensure that the maximum required working pressure of the valve can still be driven. You can ask the development department of the business unit mobile hydraulics for help regarding questions about the calculation or design of the system.

Yes, a PPRV-valve can also be used as an ON/OFF valve. In this case The valve is subjected for switching directly to the maximum current. Even without using a current control (power supply is applied 1: 1 to the valve), the valve cannot be thermally damaged under normal ambient conditions (see above). However, a disadvantage of this type of use, can be the relatively high pressure drop across the valve (up to 6 bar at 4l / min.).

For pure switching applications, it is recommended to use a DCSD04 or DCSD06 switching valve of Thomas. These valves have the identical cavity design to the series PPRV's, but have an optimized opening cross section for low pressure drop and have higher switching capacity limits.

MTTF means "Mean time to failure". The MTTF is a statistical value that is calculated by experiments and empirical estimations. It is an indication for the reliability of a component and is part of the conformity certification within the machine safety analysis.

The valves (PPCP04 and PPCP06) were constructed in accordance with the fundamental and proven safety principles according to ISO 13849-2: 2003 Tables C.1 and C.2. Thereafter, the MTTF-d value is 150 years. In order to achieve the MTTF-d value, the operating conditions of this specification must be followed and above described safety principles (ISO 13849-2: 2003 Tables C.1 and C.2) for implementation and operation must be considered.

Valve performance and technical properties

The (maximum) hydraulic hysteresis describes the highest measured pressure difference between ascending and descending pressure curve at a certain level of energization.
This means the valve is energized with a particular rate of current rising from 0 to the maximum current and then identically reverse. Thereby the operating pressure (A-pressure) above the stream is recorded in a so-called P/I-diagram. Because of the friction of the movable parts in the valve the magnetic remanence and other effects the increasing and decreasing characteristic curves are not perfectly aligned, but they are shifted in parallel.

In order to keep the hydraulic hysteresis of the valve and thus the effects in the customer system as low as possible, it is necessary to reduce the hysteresis by a suitable electrical control. By modulation of the current signal (dither) an attempt is made to put the electromagnet and thus the valve spool into vibration. If the used vibration frequency is close to the resonance frequency (of the entire system), the amplitude of the micro vibration of the mechanical components is very high and the effective friction relatively low, which leads to a reduction of the hysteresis. A common control for reducing the hysteresis is the PWM-signal (pulse width modulation), which is also often superimposed by a so-called dither signal in the high frequency range.

The Thomas valves work regular with a hydraulic pump pressure of at least 35 bar but no more than 50 bar. Within these limits pressure peaks or fluctuations in the pump pressure are not relevant to the valve performance and the set working pressure of the valve. Above a pressure of 50bar damage may occur to the O-rings of the valve sleeve and can lead to internal leakage.
For special applications with max. 350bar of static pump pressure Thomas developed a high-pressure PPRV. By the choice of another O-ring material, as well as by additional support rings on the valve sleeve, the valve with the higher pressure can be applied.

Contrary to the pump pressure the tank pressure becomes more important for the function of the valve. The tank pressure is add one to one to the working pressure. This means, that if there is a pressure in the tank connection of a valve, the characteristic curve at minimum current flow does not start at 0 bar, but at the respective tank pressure. From this offset, the pressure rises with further increasing energization in the same ratio as otherwise without tank pressure. Thus, the applied tank pressure means an offset of the characteristic curve position and possibly a pressure rise of the working connection up to saturation (working pressure ≈ pump pressure).
As a safety feature in case of failure, a further valve was developed which can withstand a higher tank pressure (static 210 bar, dynamic 50 bar). There can be no valve function at this situation, it only serves to temporarily resist a mixing up of tank connection and pump connection.

All valves by Thomas for the Off Highway market are tested according to DIN 40 050 part 9 and regardless of the installed plug they match at least protection class IP6K6 / IPX9K. Depending on the quality of the mating connector in most cases protection IP67 is also possible. For special applications, a media-tight valve was additionally developed on request of some customers that certainly meets protection class IP6K8.

All valves are equipped as standard with a surface coating that guarantees protection against rust. A thick layer passivation on zinc / nickel base protects against corrosion and is very durable at the same time. The coating guarantees a durability of 192 hours in salt spray test without red rust.
As a test basis the following standard applies here:  DIN 50962 – Fe//ZnNi(12)6//A

Through various customer requirements Thomas is committed to deliver the highest level of cleanliness and quality. For some of our customers the utmost cleanliness requirements of contamination particles out of hard substances ≤600μm is being achieved. For this purpose, the entire value chain (manufacturing suppliers, own production, storage, component handling as well as packaging and shipping) was analyzed and optimized regarding ingress of dirt. In detail even single items were changed by the construction, so that less ingress of dirt is created by the later production.

The product cleanliness is verified and monitored in Thomas’ own cleanliness laboratory. By light microscopy and SEM (scanning electron microscope) the numbers and material of particles for production lots of delivered items are determined as well as the in-house manufactured complete devices. An evaluation over weeks and months provides information about the long-term quality and problems in some processes.

Electrical actuation of the valve

There are various types of electrical control for Thomas valves (direct current, PWM, PWM + dither). The actuation can have a significant impact on the performance of the valve and also on the customer system. For example, the hysteresis and the control quality of the valve are getting better if you actuate the valve with a PWM signal (pulse width modulation) or even a superimposed dither signal instead of using direct current (DC). The vast number of customers is trying to, among other things, optimize their customer system by a special electrical actuation of the valve (see chart in "What is hysteresis?".

Digression: proportional pressure reducing valves are mainly used as pilot control valves for main pressure stages. Hereby the valves should move the main piston in the customer system by generating the pilot pressure as quickly as possible to a desired position. In order to achieve a high dynamic and short reaction time of the valve spool in the customer system, an attempt is made to eliminate the static friction of the spool in the main stage, by allowing a micro frequent oscillation of the spool in the mounting hole at any point of time. These micro-vibrations of the main spool are generated by the pressure signal of the proportional valves, which are dependent on the applied current signal. So it possible to put an oscillating current signal to the valves, so that the operating pressure pulses initiates a customer system and leads to a higher dynamic system.

The most common control of the valves is a PWM signal (pulse with modulation) in the kilohertz range, which is usually overlaid with a dither signal between 90Hz and 125Hz and an amplitude of 100 - 200 mA (peak-to-peak).

Every customer develops his own electrical control depending on application, possibilities of its control unit and texture (quality) of the mechanics.

The valve can shortly be over-energized by up to 10% of the specified maximum current without being damaged or improper operation. However. Thereby the temperature of the valve increases faster, so that at frequent over-energizing requires a higher driving voltage. This is required because the resistance of the valve increases (see also on "applicability up to which temperature"):  U= RxI

If a higher working pressure is required permanently in the customer system by the PPRV, then a next higher pressure stage of the PPRV's might be necessary. These are generally available in 20, 25 or 32 bar working pressure, in addition to some special valves.

The inductance of the coils is determined by the phase shift, which occurs by energising the coil with a defined alternating voltage at a defined frequency.

Measurements were made with 100 mV AC voltage (peak-to-peak) at different frequencies.

NG 24 Volt

20 Hz d=45 mH 1] 183,1 mH 2] 138,1 mH
200 Hzd=4,1 mH1] 76,48 mH2] 72,38 mH
500 Hz d=0,64 mH 1] 49,5 mH 2] 48,96 mH
1 KHz d=0,18 mH 1] 36,74 mH 2] 36,56 mH
5 KHz d=0,07mH 1] 20,70 mH 2] 20,63 mH

PDMV 24 Volt

20 Hz d=6,45 mH 1] 144,35 mH 2] 137,90 mH
200 Hz d= 9,58 mH 1] 79,40 mH 2] 69,82 mH
500 Hz d=6,7 mH 1] 50,40 mH 2] 43,70 mH
1 KHz d=4,93 mH 1] 36,74 mH 2] 31,81 mH
5 KHz d=1,865 mH 1] 18,560 mH 2] 16,695 mH

NG 12 Volt

20 Hz d=9 mH 1] 36,65 mH 2] 27,65 mH
200 Hz d=5,915 mH 1] 18,700 mH 2] 12,785 mH
500 Hz d=4,707 mH 1] 12,725 mH 2] 8,018 mH
1 KHz d=3,72 mH 1] 9,654 mH 2] 5,934 mH
5 KHz d=2,075 mH 1] 5,438 mH 2] 3,363 mH

PDMV 12 Volt

20 Hz d=3,86 mH 1] 36,39 mH 2] 32,53 mH
200 Hz d=3,33 mH 1] 19,285 mH 2] 15,955 mH
500 Hz d=2,39 mH 1] 12,420 mH 2] 10,030 mH
1 KHz d=1,722 mH 1] 9,048 mH 2] 7,326 mH
5 KHz d=0,588 mH 1] 4,514 mH 2] 3,956 mH

Valve versions and different individual components

Due to the modular valve system, there are already a large number of different valve variants, which are already being produced in series. Different working pressure levels, voltage variations, connector versions, valve sleeve geometries, seals, flow and leakage characteristics of various types.

Yes, there are already valve versions (PPCD04) that have loose wires with soldered connector pins. Thomas also offers to assemble tinned cable ends as well as a Deutsch- or AMP connector to the end of the cable depending on the requirements.

Currently there are no valves with a manual override function (manual operation) in the Deutsch-connector. The sum of requests has so far been limited to the AMP-connectors with manual emergency. Basically, a version with Deutsch-connector is possible, but it means increased costs and a longer validation time. For small numbers it often helps to rather put an adapter plug (mating plug Deutsch to AMP Junior Timer) to the wiring harness, which can be purchased from various suppliers.

Basically, a diode in the valve is necessary if the control unit of the customer does not include a freewheeling diode (which is very unusual). Then there is the risk, that the controller may be damaged when a certain current (0.3 joules in approximately 6ms) flows back to the controller by the inductance of the valves through the armature reset-movement in the coil when switching the valve off. Currently, there are valves with diodes already being manufactured. But these are only used as ON / OFF valves and are produced in very small quantities. The used diode is the UF54004 with up to 100V reverse peak voltage.

According to the different requirements various O-Rings are installed in the valves. Depending on the temperature, fluid compatibility and applied pressure, O-rings made of NBR, HNBR or FKM material are installed in the valves. The hardness of the materials varies depending on the application between 70 and 90 IRHD (≙ Shore A).

Depending on the configuration the O-rings can be used in a temperature range from -30°C to +180°C. By means of "support rings" pressures up to 350 bar can be sealed in the pump connection.

Most of the valves at Thomas are equipped, in their standard version, with a filter element in the pump connection (optionally this can be eliminated). Primarily it is intended hereby to prevent the ingress of dirt from the outside into the valve and also in the overall system. The mesh size of a filter may possibly affect the valve function. It can cause an undesired pressure drop or even limit the maximum volume flow. It may also lead to pulsation through the filter fabric in front of the pressure port. Depending on the nominal flow rate the filter elements have a mesh size of 128μm for PPCD04, 140μm for PPCD06 and 320μm for PPCP09 valves.

In the early days of the business unit mobile hydraulics different colors were chosen for the coil plastic for better differentiation of the valve versions in order to distinguish the 12V and the 24V coils visually.  The 12V coils are moss-grey, 24V coils are black.

There are currently 9 different mounting holes (also called cavity). On the relevant dimension drawings of the valves you also find the exact tolerated mounting hole that must be provided for assembly. The cavities are created in the customer's system by a form drill (possibly after a hole was predrilled).

Mounting hole (write and compare also in attachment to CS119)

aNG StandardTC04013
bNG Standard switching valve (PSO)TC04023
cStandard PPRVTC06023
dHigh pressure PPRVTC06033
ePPRV NG with FKM O-RingsTC04033
fPPCP06 MFTC06013
hPPCP09 HFTC09013
jPPRV NG (manifolds)TC06025
k … x   
yNG ParkerTC04063
zPPRV ParkerTC06043

Packaging, Labeling

The valves are shipped in standardized packages to all customers. 

  • PPCD04: 35 pcs with protective cap on the valve sleeve in a box with compartments 
  • PPCD06 / DCSD06: 20 pcs in a box, standing in a plastic tray 
  • PPCP09: 30 pcs in a box, standing in a plastic tray

During the manufacturing process all valves are getting as standard a labeling by a "dot pin"-marking machine. Here, the necessary information such as item number, production date, voltage version and / or pressure rating is marked on the metal housing. Depending on customer requirements, individually requested information, up to logos or QR codes with production information, can be engraved on the valve. Since the marking does not damaged the surface coating but only deforms the housing material, the designated rust protection is still ensured. In case of a complaint this type of product labeling is necessary for the traceability of the valve. Often customers over paint the valves and in case of damage the valves must be clearly identifiable after removing or scraping the color. This is being guaranteed by the dot pin-marking. Labels or slight laser markings on the housing surface often do not provide the desired sturdiness when the color of the customer is removed.

Validation tests and certifications

  • Vibrations und mechanical shock test
  • Salt spray test
  • Short circuit
  • Temperature cycling test
  • Temperature shock test
  • Endurance test (typically 10 million circuits)
  • Humidity Cycling Test
  • Burst Pressure Test
  • Pressure pulsation test

The valves of Thomas do not require a CE-label, because they have no "active electronics" implemented. Only the electro-hydraulic products, such as UPC (Universal hydraulic controller), EHA (Electrohydraulical Actuator) or I-Valve must get a CE-label and also have a CE conformity declaration (CE Certification).

Since the end of 2013, nearly all hydraulic valves are available also as ATEX-certified device versions. In this case, the valves were additionally certified by the TÜV according to the international standards IECEx equipment for use in potentially explosive environments. Further certifications according to MSHA are being planned.

The ATEX / IECEx certified devices of Thomas can be used according to the certification status in Zone 1 or 2 and 21 or 22. Further explanations regarding the safety class and the field of application, according to marking information:

Ex I : countries related to directive 94/9/EC.

Ex II 2G Ex mb (= incapsulation with high protection level for use in Zone 1 or 2) IIC (=explosion group referred to different groups of gases) T4 (=max surface temperature = 135°C) Gb (= additional information Equipment Protection Level EPL for use in Zone 1 or 2 – G=gas area).
This information is related only to usage in gas areas.

Ex II 2D Ex mb (= incapsulation with high protection level for use in Zone 21 or 22) IIIC (=explosion group referred to different groups of dusts)  T130 °C (=max surface temperature)   Db (=additional information Equipment Protection Level EPL for use in Zone 1 or 2 – D=dust area)
This information is related only to usage in dust areas.

Thomas supports the aims of the REACH and RoHS laws and attach great significance to not use conflict minerals in their products. Thus Thomas is passing resulting requirements to their suppliers before a contract is made and their information will be tested accordingly. Evidence for use of the individual substances / materials can be created and passed by Thomas to their customers.

EMC stands for electromagnetic compatibility. This technical term refers to the resistance of an "active electronic” against the influence of external electromagnetic fields. In the EMC test the products to be tested are exposed in a test to various electromagnetic fields with different frequencies, amplitudes and field strengths. Here, their function or the state of the electronics is monitored at appropriate measuring points in order to detect undesired electrical impulses.
As a test basis the following standards apply here: EN ISO 14982/2, DIN ISO 61000-4-2, ISO 7637 1-3 2000/2 / EC and CISPR 25

The pure valves from Thomas are not equipped with active electronics and must therefore not undergo EMC tests. This primarily is up to the customer with his complete system. The products with electronics (EHA, EMA & UHC, "Electrohydraulic Actuator", “Electromechanical Actuator” and "Universal hydraulic controller") are being tested closely to the standards.

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