Filtration System
To prevent premature wear, ensure that only clean fluid enters the hydrostatic transmission circuit. A
filter capable of controlling the fluid cleanliness to ISO 4406, class 22/18/13 (SAE J1165) or better, under
normal operating conditions, is recommended.These cleanliness levels cannot be applied for hydraulic
fluid residing in the component housing/case or any other cavity after transport.
Filtration strategies include suction or pressure filtration. The selection of a filter depends on a number of
factors including the contaminant ingression rate, the generation of contaminants in the system, the
required fluid cleanliness, and the desired maintenance interval. Filters are selected to meet the above
requirements using rating parameters of efficiency and capacity.
Filter efficiency can be measured with a Beta ratio (β
X
). For simple suction-filtered closed circuit
transmissions and open circuit transmissions with return line filtration, a filter with a β-ratio within the
range of β
35-45
= 75 (β
10
≥ 2) or better has been found to be satisfactory. For some open circuit systems,
and closed circuits with cylinders being supplied from the same reservoir, a higher filter efficiency is
recommended. This also applies to systems with gears or clutches using a common reservoir. For these
systems, a charge pressure or return filtration system with a filter β-ratio in the range of β
15-20
= 75 (β
10
≥
10) or better is typically required.
Because each system is unique, only a thorough testing and evaluation program can fully validate the
filtration system. Please see Design Guidelines for Hydraulic Fluid Cleanliness Technical Information,
520L0467 for more information.
Cleanliness level and β
x
-ratio
1
Filtration
(recommended
minimum)
Cleanliness per ISO 4406 22/18/13
Efficiency (charge pressure
filtration)
β-ratio
β
15-20
= 75 (β
10
≥ 10)
Efficiency (suction and return line
filtration)
β
35-45
= 75 (β
10
≥ 2)
Recommended inlet screen mesh
size
µm 100 – 125
1
Filter β
x
-ratio is a measure of filter efficiency defined by ISO 4572. It is defined as the ratio of the number of particles
greater than a given diameter (“x” in microns) upstream of the filter to the number of these particles downstream of
the filter.
Service Manual
MP1
Fluid and filter maintenance
38 |
©
Danfoss | April 2019 AX00000244en-000201
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