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An eco-friendly approach to hydraulics

By: Sven Wozniak

Agricultural engineering is one of the most innovative areas of machine and utility vehicle construction, as was clearly demonstrated at the Agritechnica in Hannover, Europe’s biggest trade fair for the agricultural industry. Here too, total costs of ownership and lifecycle management are becoming increasingly important. To put it simply: we are witnessing a trend where economics go hand in hand with the eco-friendly approach of this sector. Accordingly, it makes sense for machine manufacturers to turn their attention to hydraulic filtration systems too, in order to guarantee maximum availability for their customers in the agricultural industry.

Optimally designed filtration systems protect agricultural vehicles and prolong their lifetime

The agricultural engineering industry in Germany is currently riding on a wave of success. The mood among farmers is upbeat and investment is also at a new high. According to Hermann Merschroth, chairman of the agricultural machinery association of the VDMA (German engineering federation), ”this high is set to continue into 2008”. The latest statistics speak for themselves: the turnover of the German agricultural engineering sector for 2007 will show a growth of seven to ten percent. In figures, that represents a rise of 5.6 to 5.8 billion euros. In order for domestic utility vehicle manufacturers to profit from this trend, the high quality standards demanded by farmers need to be met.

One important aspect of this is getting the filtration technology right in their hydraulic systems. After all, this helps in no small measure to bring and keep down operating costs. Just how much influence a well functioning hydraulic filtration system can have on the service life of agricultural vehicles and equipment was demonstrated by RT-Filtertechnik at the Agritechnica trade fair. The company from Friedrichshafen has over 50 years’ experience and specialises in the development and construction of tailor-made filter systems.

Flow direction improves service-friendliness

Depending on the application, there are various factors to consider: in vehicles such as harvesting machines for example, which are operated in extremely dusty or dirty conditions, return-flow filters serve to keep the system clean. This type of filter removes the dirt that enters via the seals before it can get into and cause damage to the active components of the hydraulic system, such as pumps and cylinders.

Return-flow filters even help to prevent dirt accumulation in the hydraulic tank and so reduce the amount of servicing needed. The optimal inlet geometry of the filter head and/or inlet chambers allows the medium to stabilise, thus aiding the air bubble elimination process which takes place in the tank.

It is in the detail that RT-Filtertechnik filter solutions prove particularly innovative and service-friendly. The flow direction is always from the inside out, so that the user – i.e. the farmer in this case - can remove the saturated filter element quickly and without risk of re-contaminating the system/filter housing.

Mechanical abrasion instantly “eliminated“

Only a small number of filter manufacturers pay close attention to metal abrasion in hydraulic systems. Through the use of magnetic separators – also known as filter cores – which are integrated in RT filters as standard – this potential source of wear can be simply “deactivated”. That way, the farmer is assured of a magnetic (pre-filtration] system which protects all running surfaces in the cylinder, pumps and valves against fine metal chips – and with a minimum of fuss. This prolongs the lifetime of the hydraulic system and increases the durability of his vehicles and equipment.

The return-flow filtration can be configured as a full-flow system. To ensure the required flexibility, vehicle manufacturers offer a choice of installation options for these filters, i.e. mounted on the tank with a filter head or integrated inside the tank as a space-saving solution. The tank-integrated version is the cheapest system solution for larger flow rates, however, offering filter grades as fine as 5 μm(c).

Because the movements of agricultural equipment and vehicles are increasingly controlled via proportional valve technology nowadays, the importance of microfiltration is growing. This is achieved by the manufacturers – and users - of agricultural vehicles via so-called combination elements. RT-Filtertechnik has developed filter elements of this kind specifically for higher-end applications without having to change the existing filter concept. These elements are designed to fit the existing filter housing and are simply inserted.

The two-stage structure of this filter element ensures that large particles are filtered out more quickly than small ones, thus preventing the filter element from building up a high flow resistance. With the continuous passage of hydraulic oil through the filter elements, the ultra-fine particles are also retained successively by the fine-meshed part of the “two-stage filter”. The resulting effect in the hydraulic system is comparable to a certain extent with that of bypass filtration – but without the need for any further alterations or design modifications.

The suction/return-flow filter as a multifunctional solution

The suction/return-flow filter was developed by RT-Filtertechnik as a modern filter version with a high level of functional integration. This type of filtration is ideal for all applications where space is limited, e.g. in yard cranes. It was designed for vehicles in which both the working movements and drive are hydraulically actuated, e.g. telescopic loaders.

The hydrostatic drive in a closed circuit must be equipped with special protective measures to ensure smooth operation over a long period. Otherwise certain phenomena will occur - particularly at low temperatures, when the hydraulic oil is still very viscous – resulting in insufficient oil getting through to the units.

As vehicle manufacturers know, in this situation it is vital to ensure optimal suction conditions upstream of the pump in the feed pump circuit. For this reason, the suction/return-flow filter has a pre-charge pressure of 0.5 bar, so that the required oil is actively made available to the system. The return-flow oil filtered by the full flow method is subjected to this pre-charge pressure and fed directly into the suction connection. The surplus volume is fed to the tank via a pre-charge valve.

Avoiding dry run damage

Should a shortfall nevertheless occur in the oil supply due to extremely low flow rates, reliable replenishments are guaranteed by an anti-cavitation valve, which can be equipped with a protective strainer, thus preventing dry run damage. This filter technology enables farmers to dispense with the inevitably time-consuming warm-up phase. The vehicle or machine can be put into operation immediately – even after a so-called “cold start”, thereby saving precious time.

An additional pressure limiting valve in the filter housing prevents the pressure from rising above the permitted level in the return-flow system. As a further advantage, the two-in-one concept of the suction/return-flow filter saves the vehicle manufacturer not just space but also the installation costs of the pipes and conductors that would otherwise be needed for an additional filter.

In order to avoid cavitation damage to the pump, the vacuum in the suction line should not be allowed to fall below -0.25 bar rel. In practice, it is also important to make sure that the supply lines – i.e. the suction sections – are as short and large as possible. Another tip from RT-Filtertechnik is to limit the flow rate to a maximum of 1 m/s as a general rule.

Suction filters - an indispensable solution

In vehicles and harvesting machines where the hydraulic tank is integrated in the vehicle frame, the tank-integrated type of suction filter offers a central solution for oil purification. It is fitted directly upstream of the pump by way of a preliminary or coarse filter. The optimised flow technology of the housing and element keeps pressure losses to a minimum and ensures high dirt absorption capacities. This filter is available for flow rates of up to 700 l/min. Such units are also attractively priced and are generally part of the standard equipment whenever high-quality pump systems are involved.

The special design of the suction/return-flow filter allows multiple functions within the same housing and is particularly suitable for use with hydrostatic drives.

These filters are available in tank-integrated and tank-mounted versions. Moreover, both types can be installed either horizontally or vertically, which gives the designer much greater flexibility. Thanks to the combination of an integrated inlet valve and a special drainage device, the element can be changed in a leak-free environment without emptying the tank, even in a horizontal setting. Here again, as with all other filter types from RT-Filtertechnik, the flow direction from the inside out eliminates the risk of re-contaminating the system when changing the filter and also offers the possibility of magnetic pre-filtration via a magnetic core.

“Police filters“ in dirty environments

The suction filters can be equipped with recyclable wire mesh elements or plastic or glass-fibre-based elements. The standard elements in this series are made of PE. For safety reasons, none of the suction filters from RT-Filtertechnik have a bypass valve and should therefore be protected via contamination indicators.

Besides the individual configuration of filter systems around the pump and hydraulic tank, additional “police filters” can be added wherever a high level of incoming dirt is expected. As line filters, these perform the function of full-flow filtration; as safety filters, they maintain a constant level of oil purity throughout the system. This additional feature is useful in highly ramified and extensive hydraulic systems. They also keep the oil clean and protect dirt-sensitive components such as proportional, servo and control valves, etc.

The advantages of high-efficiency magnetic pre-filtration and quick element replacement without re-contamination at the clean oil end are standard in these products too. Depending on the pressure fluid, a choice of different glass-fibre fleece combinations and wire mesh elements is available.

Filters essential for filling and ventilating

Filling and ventilating filters may seem rather mundane, but they are extremely important. Though they are often paid little attention, they play a key role in keeping dirt out. Designers need to be aware that the fluid movements in the hydraulic system cause the tank to “breathe”. The resulting air exchange via the ventilation socket encourages the entry of solid particles and moisture. It is therefore advisable to fit a ventilating filter right from the start.

The same principle applies when filling the system with fresh hydraulic oil. Normally, hydraulic systems are flushed and filled via specially constructed filling trucks. These are rarely available in the field, however, so the easiest way to top up the hydraulic oil is via the tank filler neck. A filling filter fitted here prevents the entry of unwanted solid particles.

The “health tips“ for a long-lived hydraulic system are many and varied. Which combination is best depends on the individual application. One thing is certain, however: the reliability and lifetime of agricultural vehicles and equipment are increased by a comprehensive filtration system.

Dipl.-Ing. Sven Wozniak
Manager technical customer advice
RT-Filtertechnik GmbH

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