four functions of a hydraulic fluid
- transmit power
- lubricate
- seal clearances
- provide cooling
effects of high temperature
- oxidation of the oil
- formation of insoluble gums, varnishes, and acids
- deterioration of seals (they harden and leakage begins)
- loss of lubricity
- changes in viscosity
lubricity
the ability of the fluid to maintain a film between moving parts
indicator of high oil temperature
- it will have a darker color and an odor of scorched oil
- heat-peeled paint on the surface of the components or reservoir
sources of heat generation
- friction
- undersized hoses/reservoir
- fluid returning at high pressure over the relief unnecessarily
three types of heat exchangers are used to cool hydraulic oil
- shell-and-tube
- air cooled heat exchanger
- plate and coil
three types of failure
degradation, intermittent, and catastrophic
degradation
the performance of the component degrades over time as surfaces wear, clearances increase, and leakage increases
intermittent
valves stick and then break loose such that operation is intermittent.
catastrophic
catastrophic failure occurs when a major component breaks apart. Often, debris causes the failure of other components, and a total replacement of the circuit is required
four sources of contamination in hydraulic fluid
built-in contamination, contaminated new oil, ingressed contamination, and internally generated contaminate
built-in contamination
this is contaminate that was left in the system when it was assembled
contaminated new oil
contaminate is introduced during the manufacture and subsequent handling of oil
ingressed contamination
this contaminate can enter with air flowing into the reservoir through the breather cap, or it can ride in on a cylinder rod
internally generated contaminate
particles removed from the interior surface of the components will circulate in the system until they are removed. Each impact of one of these particles with a surface causes more damage. This phenomenon is known as the wear regeneration cycle.
internally generated contamination causes damage in the following ways
- abrasive wear: particles are hard, bridge across the clearance between two moving surfaces and abrade one or both surfaces
- adhesive wear: moving parts adhere, or stick together, and damage results
- fatigue: stress causes a crack which causes the surface to fall apart
- erosive: particles erode the surface away
- cavitation: shock waves from cavitation hits surface and cause damage over time
- corrosive: chemical attack of material causes damage
the current international standard for cleanliness of a hydraulic or lubricating fluid
ISO 4406: specifies a laboratory particle counting procedure to determine number and size (in microns) of solid particles in a milliliter of fluid
solutions to heat generation
- pump unloading (tandem center, selection of pressure, hi-lo system, pressure comp pump)
- use less tees/crossings b/c of pressure drops
- place values in system sized for max flow
high temperature
temp higher than 160F causes a break down of fluid
-solution: heat exchangers
clean fluid requires filtration. different ratings are
- GPM rating: sized for max flow rate
- psi rating: can undergo negative pressure
- micron rating: cellulose (paper filter) and mesh (wire filter) [40 micron to 5 micron]
- beta rating: ratio of particles
silting
very small particles are forced into clearances by pressure and blocks the valve
a number of factors are considered in setting a target cleanliness level
- components in a system
- fluid
- start-up temperature
- duty cycle
- system design life
- cost of production interruption
- safety
4 types of maintenance
- none
- preventive (set schedule)
- corrective (fix as needed)
- predictive
where to place filters
- before the pump (suction strainer/filter)
- after the pump to protect expensive components such as electric servo valves and proportional valves
