9 - AHP Merkle GmbH

1.11 Effect of external forces
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Hydraulic cylinders are extremely powerful, whose specifi c force generation is practically unique compared
to other types of actuators. They provide their performance in the axial direction. This means that it is necessary
to calculate the buckling strength and the basic system limits based on the pulling and pushing loads.
At the same time, interaction with the specifi c application almost always results in side forces. These should
be eliminated as much as possible, or should be absorbed using separate (mechanical) construction; this is also
required by DIN EN 982. For example, one good possibility is to use appropriate couplings, like those offered by
AHP Merkle. These allow side motions without transferring them to the piston rod.
Alternatively, AHP Merkle has cylinder options that can absorb transverse forces and torques, such as the push
units (BSE, ZSE) and core pull units (KZE).
Side forces or torques on hydraulic cylinders lead to:
● damaged guides
● damaged piston rods
● damaged running surfaces
● destroyed seals
1.12 Hydraulic fluids
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Hydraulic fl uids are subdivided into the following basic categories
● Mineral-oil based hydraulic fl uids
● Flame retardant hydraulic fl uids
● Quickly biodegradable hydraulic fl uids
Mineral-oil based hydraulic fl uids are designated in ISO 6743/4 as HL, HM, HV, and in DIN 51524 with HL,
HLP, HVLP.
HL stands for hydraulic oils made of mineral oils with active ingredients that improve their corrosion protection
and aging resistance. HLP oils improve the corrosion resistance, the aging resistance and the wear due to
scoring in the mixed friction range. HVLPs additionally improve the viscosity/temperature behavior. There are
also HLP-D hydraulic fl uids that include cleaning additives (detergents).
There are certain additives in mineral oils that can accelerate the aging of elastomer
seals at high temperatures. This results in subsequent vulcanization, which causes
hardening and a loss of elasticity.
Use of a coupling to avoid side loads.
Flame retardant hydraulic fl uids are classifi ed in VDMA 24317. They are available as HFAE, HFAS, HFB, HFC
and HFD oils.
HFAEs are oil-in-water emulsions with a water content of greater than 80% and a mineral-oil or soluble
polyglycol-based concentrate. In the mineral-oil based option it is necessary to watch out for decomposition
and microbe growth. The fl uid can be used at temperatures from +5 °C to +60 °C (+41 °F to +140 °F).
For HFASs with synthetic concentrates there is no danger of decomposition. However, attention should be
paid to the signifi cantly increased tendency for corrosion.
HFBs are water-in-mineral oil emulsions with a water content greater than 40%. These hydraulic oils can
also be used from +5 °C to +60 °C (+41 °F to +140 °F), but are not approved for use in Germany due to their
inadequate fi re safety characteristics.
HFCs are so-called aqueous glycols, quasi aqueous monomer or polymer solutions (frequently polyglycols).
Their water content is generally between 35% and 65%. These fl ame resistant hydraulic fl uids can be used at
pressures of up to 250 bar (3625 PSI) and temperatures between -20 °C and +60 °C (between -4 °F and +140 °F.