Hydraulic design priorities

If you have a load that you need to move then there are many options to consider.

Most of the major equipment suppliers offer a range of hydraulic, pneumatic, electrical or electromechanical drive systems. You should not assume one is better than the other but instead, define your load conditions, then analysis the different options to determine which will be the most appropriate. The equipment suppliers will usually help their customers with this selection process as it is also in their interest to identify the best performing solution.

There is no one approach that will be the best in all situations. For example, most smaller automotive fan drive systems are direct mechanical or electrical drives as they give low cost, efficient drives. However, a major benefit of hydraulic fluid power systems is that the power source can be located remotely from where the power output is required. Therefore on large mobile excavators, it may be more efficient to have a cooling fan driven directly from the engine drive shaft, however, this limits the fan location and blade clearance required to compensate for engine vibration. The small loss in system efficiency from using a hydraulic drive is more than compensated for by the increase in overall cooling efficiency. And there is rarely space for an electric motor to drive the fan in the ideal location, on a compact mobile machine.

Users do need to be cautious about change. In the past, some industries have tried to move away from hydraulic drives to electrical ball and screw. From a performance point of view this may have been worthwhile but over the extended service life, we find that the tiny levels of airborne dust tend to wear the mechanical mechanisms, creating discontinuities or backlash in the drive. Within the traditional hydraulic drives, the power would always have been transmitted via a contact-free, hydrostatic bearing faces so there would be no mechanical contact and thus no wear. Therefore the working life is greatly extended.

A more recent development that is again replacing some types of hydraulic drives is high voltage, variable frequency drives. A typical example would be replacing hydraulic motors on the drive wheels of large forklift trucks.

There are a large number of market sectors that all use slightly different hydraulic components. For example; mobile, industrial, automotive, aerospace, presses, marine and many more.

Hydraulic equipment suppliers generally offer different products for each market, based on their experience of what works best. Their websites generally direct you to a market sector before directing users to buy directional or pressure control valves for example. This approach is quite understandable because users don't appreciate the performance requirements for every sector and with so many different valves options available, would probably select the wrong ones if they had a choice.

We recommend that, wherever possible, designers/maintainers of hydraulic equipment stay with this approach and only select components that are standard within their market sector. This is because, while the main overriding hydraulic principle will remain the same for each sector, the individual design priorities are likely to be very different. See the priorities review below for some generalised examples.

The following slides provide some rough guides for the design priorities in different market sectors. You must always ensure that the components you specify are appropriate for your application, however, please be aware that the advice given in the remainder of this training may only apply to the application being discussed, rather than all hydraulics.

The design life or duty of a hydraulic system can vary greatly. 90% of all hydraulic failures are due to contamination so designing a system that starts and remains clean is vital.

Mobile applications can be simple and robust but sometimes lower cost and therefore quality.

Industrial systems may be better quality for longer life but can be more complicated and therefore less reliable in more arduous environments.

Hydraulic system costs will vary significantly. Probably the most important factors are production volume, environment, life cycle duty, market and quality of control.

The external environmental conditions around equipment will have a major impact on the component design. Industrial applications are generally dry and reasonably clean, while mobile hydraulics will normally get very wet and muddy. Marine application with salt water and extreme temperatures are some of the most arduous.

Many applications just need to move actuators with directional valves that are either open or shut.

Proportional valves allow actuators to be accelerated more smoothly or driven at a controlled speed.

High performance proportional and servo valves can provide a much faster response with better resolution and less hysteresis etc.

With any type of control, the system's natural frequency will need to be compatible with the valves selected.

Industrial design techniques offer reliable 'right first-time' design.

Mobile design is generally closer to the maximum limits of the equipment and the wider operating ranges need field development trials to make sure the design is right.

There are two main groups of hydraulic valve design. Poppet valves tend to have very low leakage usually only measured as visible drops per minute or cylinder movement per hour. Spool valves will always have some clearance between the spool and body and this will always result in a leakage in the order of 30 cc/min, although this will depend on fluid temperature and pressure.

Initial cost can be kept low by making systems that just do one job and don't include additional features to make servicing and repair much easier.

Through life running costs can be reduced with more robust valves or increasing equipment availability with easy to install spares.

The life and reliability of hydraulic equipment will be affected by the level of vibration, internal loadings, single shock events etc.

How much abuse it has to withstand from the operator is also important.

Another key factor is the type of hydraulic fittings used and how well they can withstand abuse.