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Energy Efficiency & Integration: Return-Suction Filters

Simple and exact design for maximum customer benefit

Ruecklaufilter

Closed circuit with suction filter and open circuit with return filter

Closed circuit with suction filter and open circuit with return filter

Closed circuit with pressure filter and open circuit with return filter

Closed circuit with pressure filter and open circuit with return filter

1 Variable pump of the closed hydrostatic drive, 2 Filling pump of the closed hydrostatic drive, 3 Working pump of the open hydraulic circuit, 4 Hydraulic motor of the closed hydrostatic drive, 5 Drain line, 6 Cooler, 7 Return line, 8 Return-suction filter, 9 Suction line

1 Variable pump of the closed hydrostatic drive, 2 Filling pump of the closed hydrostatic drive, 3 Working pump of the open hydraulic circuit, 4 Hydraulic motor of the closed hydrostatic drive, 5 Drain line, 6 Cooler, 7 Return line, 8 Return-suction filter, 9 Suction line

EXAPOR MAX 2

EXAPOR MAX 2

Section through return-suction filter E 158

Section through return-suction filter E 158

Funktion 2

Particularly in self-propelled machines that are equipped with a hydrostatic drive and combined working hydraulics, the filter concept furthermore often consists of a return filter (working hydraulics) and a suction or pressure filter (hydrostatic drives). The filter finenesses have to be co-ordinated so that the cleanliness classes 20/18/15 … 19/17/14 (acc. to ISO 4406) demanded by the drive manufacturers will be reached.

Disadvantage of a closed circuit with suction filter and open circuit with return filter:
Due to the high kinematic viscosity of the hydraulic fluid at low temperatures, suction problems arise especially in the cold start phase.              

Disadvantage of a closed circuit with pressure filter and open circuit with return filter:
The pressure filter protects the axial piston pump in the closed circuit, but the filling pump is not protected. Suction problems still arise in the cold start phase. 

Return-suction filter concept
The use of a return-suction filter offers far-reaching functional improvements. In machines with a hydrostatic drive and combined working hydraulic system, return-suction filters replace the suction or pressure filters required for the feed pump of the closed-loop hydrostatic drive circuit as well as the return filter for the open-loop working hydraulic circuit. Compared to these filter concepts, often also a cost reduction can be achieved.

Conceptional advantages

  • Only one filter for both circuits
  • In both circuits the total oil volume is filtered in the return line
  • Continuously more oil is flowing in the return line than it is taken off via the suction line
  • The feed pump sucks pre-charged oil via a pressure control valve – therefore excellent cold start behaviour
  • The feed pump is always supplied with filtered oil. No bypass available (except for in-line versions). A pressure relief valve is installed to protect the shaft seals against overloading.

While each circuit operates independently with separate filters, the combination of the two circuits via the return-suction filter causes interaction between the circuits. If the design criteria described below are taken into account, you can take full advantage of the benefits provided by the return-suction filter concept.

Required return flow in the system
In order to maintain a pre-charging pressure of approx. 0.5 bar at the intake of the feed pump, the return flow must exceed the suction flow under any operating condition, varying depending on the filter size and the design.

Permitted flow rate of the feed pump

  • At operating temperature and rated speed, the volume flow of the filling pump should not exceed 50 % of the nominal flow rate of the filter (e.g. with a return-suction filter with a nominal volume flow / return flow of 200 l/min, the volume flow of the filling pump should not be higher than 100 l/min).
  • At extreme cold starts (n = 1000 mm²/s) and slightly increased idle speed (n = 1000 min-1) the volume flow of the filling pump should be 20 % less than the nominal flow rate of the filter.

Normal operating conditions
The hydraulic oil returning from the circuit (A) passes the filter element (1), is pressurized by a 0.5 bar check valve (2) and supplied to the feed pump of the hydrostatic drive (B). The surplus oil between return and suction volume flows filtered into the reservoir. The pre-charging of 0.5 bar in the suction line minimizes the risk of cavitation in the filling pump and allows full performance even during the critical cold start phase. In normal operation, a lack of oil is excluded.

The integral pressure relief valve (3) prevents too high back pressure in the return line. As this valve leads the oil directly into the tank there is no connection between the return line (A) and the connection of the feed pump (B) (no bypass). The emergency-suction valve (4) with protection strainer (5) supplies the feed pump in case of a short term of lack of oil (ventilation/cold start).

Pressure loss in the suction lines
Under above mentioned cold start conditions, the pressure loss in the suction lines must not exceed 0.4 bar. This ensures that the filling pump is supplied with sufficient oil even with partly contaminated filter element.

Backpressures in the system return lines
When the drain oil from the hydrostatic drive is routed across the filter, the permitted leakage oil pressures have to be observed in order to protect the shaft seals. Here, besides the backpressure of the filter, also the pressure loss caused by the leakage oil pipes and the oil cooler are to be considered. Depending on the application, the use of a cooler bypass valve is recommended.

Filter fineness and oil cleanliness grades
As standard, for all return-suction filters two ARGO-HYTOS filter grades are available: 10EX2 and 16EX2. Both with the patented EXAPOR®MAX2 design.The following oil cleanliness according to ISO 4406 can be achieved:

10EX2    18/15/11 … 14/11/7

16EX2        20/17/12 … 17/14/10

The manufacturers of hydrostatic drives mostly recommend for normal operation an oil cleanliness of 20/18/15 as well as of 19/17/14 for increased requirements (t > 90 °C).

Even with the filter fineness 16EX2 these requirements are met by 100 %.

The structure of the specially developed 3-layer filter material, using glass and polyester fibers of different finenesses combined with an improved hybrid support fabric (patented) made of stainless steel and polyester sets the standard for:

  • Pressure loss
  • Dirt holding capacity
  • Flow fatigue stability

The plastic sleeve used offers the following benefits:

  • Custom label
  • Protection from damage
  • Improvement of flow fatigue stability

For the user, these improvements bring:

  • Extended service intervals
  • Higher operational reliability
  • Improved oil cleanliness
  • Increased performance
  • Positive element identification
  • Reduced operating and maintenance costs

As an alternative to EXAPOR®MAX2, the newly developed and less expensive EXAPOR®Light filter elements are available on request.
Due to reduced dirt holding capacity, they are especially suitable for systems with less stringent requirements (e.g. approx. 500 operating hours / year).

Available filter series
Depending on the mounting position and the required performance, at ARGO-HYTOS one can choose between the following designs and series:

DesignSeriesNominal Flow Rate
Inline mounting  E 068 / E 088Up to 100 l/min
Inline mountingE 178 / E 258 Up to 250 l/min
In-tank mounting E 084 Up to 80 l/min
In-tank mounting E 158 / E 198 / E 248 Up to 250 l/min
In-tank mounting E 328 / E 498 Up to 600 l/min
In-tank mounting E 598 / E 998 Up to 850 l/min

The in-tank mounting versions are equipped with pressure relief valves; the in-line mounting variants are equipped with a bypass. The modular design of the filter allows for easy implementation of customer wishes such as e.g.  special connection configurations.

Summary
In the design of return-suction filters, the specific requirements of hydrostatic drives have even more to be taken into account than in the design of separate filters for the filling pump and the working circuit.

The advantages of the return-suction filter concepts

  • Excellent cold start characteristics of the system, as the filling pump is supplied with pre-charged oil (reduced risk of cavitation)
  • Protection of the filling pump by supplying with micro-filtered oil
  • Fewer components by saving a filter
  • Reduction of spare parts and maintenance costs come with exact design into their own, thus ensuring maximum efficiency and performance of the system even under extreme operating conditions.

 

Technical Data

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