www.energyabsorption.com
Revision B June 2013
www.highwayguardrail.com
41
All rights in copyright reserved
Hydraulic Fluid
1. Purpose
The main purpose of hydraulic fluid is to transfer power from the pump to the actuators.
Secondary tasks include reducing metal on metal friction and preventing the formation
of rust and sludge. These traits are achieved when the oil has a good lubricity
(slipperiness), and with additives that inhibit oxidation and the entrainment of air and
water in the oil, respectively. The viscosity (thickness) of the oil
must be appropriate for
the operating temperature to minimize unwanted leakage and to lubricate the close
fitting parts in the system.
Additionally, the oil must be compatible with the seals used in the system. Finally, the
oil must be able to pour or
fl
ow at the lowest expected operating temperature so that it
can enter the reservoir and pump for use. For all of these reasons, automatic
transmission
fl
uid (ATF, Dexron
®
III) has been found to be the best readily available
fl
uid for the job in most climate conditions.
2.
Selecting Fluids for Applications Outside ATF’S Temperature Range
When looking for fluids that can be used in place of ATF or for applications where the
operating temperature is outside the range of ATF, the following specs should be
discussed with your local oil distributor:
A. Fluid must be compatible with Buna-N sealing compounds.
B. The Pour Point must be below the lowest anticipated temperature that will be
encountered.
C. It should contain Rust or Oxidation inhibitors as well as other detergent type
inhibitors.
D. The viscosity (SUS) shall lie between 80 and 375 in the operating range, with the
ideal viscosity near 200 SUS.
E. The viscosity index should be as high as possible. As an example, ATF has the
following specs as listed by most oil manufacturers:
[1] Viscosity
(SUS)
37 deg. C [99 deg. F] 185 to 205
99 deg. C [210 deg. F]
45 to 55
Pour Point
-44 deg. C [-47 deg. F] to
-37 deg. C [-35 deg. F]
Viscosity Index
145 to 165
Note:
In an emergency, for cold weather applications, SAE 10 W non-detergent oil mixed by
volume with no more than 30% #1 fuel oil or kerosene can be used.
