turbulence, and hence lower pressure drops, than right-angle fittings, but the space available will normally dictate which approach is most feasible.
As mentioned in Chapter 7, the installation of flexible hoses has to be carried out with great care to ensure that:
• their bend radius is above the minimum specified by the hose manufacturer under all operating conditions
• they cannot rub or chafe against other hoses or components during operation
• they are not installed twisted along their length and cannot twist axially during normal operation
• the correct combination of hose and fittings has been used.
It is normally necessary to fill the cases of pumps and motors having external drain connections with clean fluid before they are started, in order to provide initial lubrication for their moving parts (Fig. 8.3). This should be done even before the electric drive motors are jog-started, to check whether the direction of rotation is correct. A pump started dry can be damaged in just a few revolutions, thereby shortening its useful life. Once operating, internal leakage will ensure the cases are maintained full of fluid, provided the drain connections have been piped correctly.
COMPONENTS ARE DISTRIBUTED AROUND THE VEHICLE AND CONNECTED WITH FLEXIBLE HOSES
Fig. 8.2 Typical mobile system layout (Image courtesy Eaton Corp.)
FILL PUMP AND MOTOR CASES WITH CLEAN FLUID
Fig. 8.3 Prefill pump and motor cases with clean fluid (Image courtesy of Eaton Corp.)
TOP TIP
Although they often appear similar, flexible hoses come in many different types and ratings, so always refer to the manufacturer’s data sheet for information on the correct hose assembly and fitting.
If fitted, accumulators can then be pre-charged with nitrogen to the required pressure (Fig. 8.4). This will normally involve connecting the accumulator to a nitrogen bottle via a charging kit, and opening the bottle tap, accumulator charging valve and the accumulator fluid drain valve. The accumulator bag can then be charged with gas until the required pressure is achieved. In some cases, two or more nitrogen bottles may be required.
Fig. 8.4 Pre-charging an accumulator
ENSURE DRAIN VALVE IS OPEN WHEN PRE-CHARGING
THE ACCUMULATOR
Whatever type of system is being installed, keeping the installation as free from contamination as possible is vital. Whereas it is possible to control the environment of factory-assembled mobile machines, this is often much more challenging when installing industrial systems. Where other construction work is going on at the same time, dirt and moisture in the atmosphere tends to find its way into any exposed openings in components, pipes or hoses and, as explained in Chapter 4, such contamination can cause serious problems.
There are, therefore, two aspects to achieving a clean installation of a hydraulic system on a machine: preventing dirt from entering the system (prevention) and removing any dirt that has entered before the system is put into operation (cure).
While prevention may be the best approach, it may never be 100% possible to keep all contamination out of the system during installation, and in practice both methods are usually required. However, placing greater emphasis on prevention will make the subsequent removal of contamination much simpler.
Preventing entry of contamination
It can generally be assumed that hydraulic components purchased from reputable suppliers are clean to an acceptable level and supplied with all ports plugged or capped by some means to prevent the entry of dirt. Screwed ports, hoses and pipework are normally fitted with plastic caps, while gasket-mounted components have shipping plates attached to them (Fig. 8.5).
These protection devices should remain securely attached until the last possible moment (i.e. just before the component is installed in its final location). The same also applies to fittings and other small components, which should remain sealed in their packaging until just before they are used. Similarly, any openings in reservoirs should
TOP TIP
It is sometimes stated that removing contamination from a hydraulic system is twice as difficult as preventing it from entering in the first place.
POINT OF INTEREST Note that most manufacturers recommend that bag-type accumulators are mounted vertically to avoid damage to the bag caused by the closing of the poppet valve when draining down.
The initial fill of fluid and any subsequent top-ups should therefore be made via a suitable filter or via a transfer cart (see Fig. 7.14).
remain closed until the final connections have been made. Wherever possible, sub- assemblies, such as manifold blocks and reservoirs, should be assembled in a clean environment where airborne contamination can be controlled.
It is often assumed that new hydraulic fluid is clean enough to be used in a hydraulic system. Experience has shown, however, that this is rarely the case, as illustrated in Fig. 8.6.
KEEP SHIPPING PLATES ON COMPONENTS UNTIL READY
FOR USE
SEAL RESERVOIR OPENINGS UNTIL CONNECTIONS ARE MADE CAP PIPES AND
HOSES UNTIL READY FOR USE
KEEP FITTINGS IN SEALED BAGS
Fig. 8.5 Keep components sealed (Image courtesy of Eaton Corp.)
ISO 22/21/18
NEW OIL DELIVERED IN DRUMS
ISO 17/16/13
ISO 19/18/15 ISO 15/14/11
REQUIRED BY MODERN HYDRAULIC SYSTEMS
Fig. 8.6 Comparison of fluid cleanliness levels
Removing contamination
Once the system has been installed it will inevitably require some degree of flushing to remove inbuilt contamination before it is put into service. However, it may be possible to remove some of the contamination generated during the installation process even before flushing is undertaken.
Where flexible hose assemblies are being manufactured on site, contamination particles will inevitably be generated by cutting the hose. A possible way of removing such contamination is to use a plastic foam pellet ‘fired’ through the hose by means of compressed air (Fig. 8.7). The tightly fitting pellet then pushes out contamination as it is forced along the hose. The process is normally carried out at least twice (once in each direction), or as many times as is necessary to remove the contamination.
Care must be taken, however, to ensure that sharp edges on the end fittings do not
‘shave off’ particles from the foam pellet and thus create further contamination. If in doubt, the process should be carried out on the hose alone (i.e. without end fittings), and then the hose assembly flushed through with fluid once the fittings have been attached. As before, if the hose assembly is not used straight away the ends should be capped and, ideally, the hose labelled accordingly.
1. CHOOSE APPROPRIATE SIZE AND TYPE OF PROJECTILE
2. APPLY AIR NOZZLE ENSURING AIR SUPPLY IS DRY AND CLEAN
3. BLOW PROJECTILE THROUGH HOSE OR PIPE.
USE ABRASIVE PROJECTILES TO REMOVE SCALE, ETC.
4. REPEAT PROCESS IF NECESSARY.
ONCE CLEANED, CAP PIPES AND HOSES UNTIL READY FOR USE
Fig. 8.7 Cleaning flexible hoses
The same process can be used for straight solid pipes. If there is any corrosion evident in the pipes, an abrasive pellet can be pushed through to remove the corrosion, followed by a cleaning pellet to remove the contamination. However, the best solution is to use clean pipes that are free from corrosion.
After the installation is complete, but before the machine is put into operation, most hydraulic systems will need to be flushed to remove any last traces of installation contamination. How rigorous this process needs to be will depend largely on the
target cleanliness level of the system, which in turn will be determined by such factors as:
• the sensitivity of the system components to contamination
• the type of fluid being used
• the operating pressure and temperature levels of the system
• the criticality of the machine operation.
Although the flushing process only takes place at low pressure, it is often recommended to remove very sensitive components (such as servo or high- performance proportional valves) from the system while flushing takes place.
Normally, such components are manifold or sub-plate mounted, so manufacturers often supply special connection blocks that can be fitted in their place while system flushing takes place.
In order to create a continuous flushing loop, cylinders should be bypassed by a temporary connection across their ports (Fig. 8.8). The same may also apply to motors if they are likely to be damaged by contamination being flushed through them, albeit at low pressure.
DEFINITION
The target cleanliness level is the level of cleanliness required to ensure satisfactory life of the hydraulic system. It can be determined at the design stage of a system, and is dependent on operational factors and the type of components used.
ACTUATOR CONNECTIONS
LOOPED
ADDITIONAL RETURN-LINE
FILTER (IF REQUIRED) FLOW CONTROLS
FULLY OPEN
ADDITIONAL LOW-PRESSURE, HIGH-FLOW PUMP (IF REQUIRED)
Fig. 8.8 A typical flushing arrangement
The process of flushing requires a high flow velocity through the system, to create as much turbulence as possible in order to remove contamination particles from every corner of the installation. Whether or not a flow is likely to be turbulent can be estimated by calculating a factor known as the Reynolds number. Generally speaking, the higher the flow velocity and the lower the fluid viscosity the more likely it is that turbulent flow will result.
In practice, this will mean ensuring that the flushing fluid is fully heated to the maximum practicable temperature and that as much flow as possible is passing
through the system. If the normal system pump does not create sufficient volume on its own to generate turbulent flow, additional pumps may be required on a temporary basis to increase the flow velocity. Alternatively, if standby pumps are incorporated in the system, these could be used to augment the normal system pump flow. Similarly with filters, additional large-capacity flushing filters can be temporarily incorporated in the return line of the system if the normal operational filters are not suitable.
Fluid samples should be taken at regular intervals during flushing, and the flushing process continued until the target cleanliness level has been achieved. Wherever possible, the fluid used to flush the system should be the same fluid that will be used when the machine is put into operation. In this way, at the same time as the system is being cleaned by the flushing process the fluid is being cleaned as well.