Wherever I go in the world of road transport I never fail to be impressed by hydraulically-powered material-handling equipment fitted to trucks and off-road machines. The work they do is quite amazing. The problem is that multi-million-Rand vehicles and equipment can stand due to simple hydraulic failure. Worse still is that hydraulic performance is part of a logistical chain - if the loader doesn't work the trucks stand and the project comes to a grinding halt. In today's world we 'sweat the assets' - just ask ESKOM about that - and there's no room for downtime because of ignorance over hydraulics writes FleetWatch technical correspondent Dave Scott

Like with most problems in transport it starts with price before cost of ownership. Truck body builders are profit driven in a competitive market so a 'United Nations' of components is sourced from around the globe. Chinese components are matched to Turkish and Italian hydraulic parts without too many calculations and reliability tests being done. The result? It turns into on-site failure far away from service support with massive blame-assignment recriminations. But of course, like UN delegates, everyone points fingers at everyone else but themselves.

And then ignorance takes over. To speed up the rates at which hydraulics work, technicians turn up the pressure in systems, mistakenly believing that it delivers faster results. All that screwing up pressure achieves is lifting more load than the rated system capacity with a knock on effect of blowing a seal, or worst by blowing/bulging the cylinder. In most cases, this means cylinder replacement, a broken hydraulic hose or no payload movement if the pressure relief valve is tampered with and incorrectly set. Faster work comes from hydraulic flow rate delivered from a pump.

"Pumps do not create pressure - it's overcoming the resistance to the load to be lifted or tipped that creates hydraulic system pressure," says Garnett Cross, a well-known South African hydraulics expert. And an over-size pump causes cylinder damage where the excess torque developed can also damage a gearbox to which it is connected via a power-take-off unit.

It is important that the entire system be full of liquid. Liquids are considered to be incompressible whereas gases are considered to be compressible and hydraulic systems will fail to operate properly if air is present in the lines or cylinders. Understanding the relationship between an applied force and the result obtained, is a key aspect of hydraulic machines. In practice, Pascal's Law allows for MPa 'push' to be multiplied many times into MPa 'force' that delivers a mechanical advantage and is why hydraulic machines can do so much work to make them an indispensible part of our lives.
 

Scrap metal operations Metal recycling and scrap metal recovery depends entirely on reliable hydraulic machinery. It’s one of the highest variable cost factors in daily maintenance and downtime.

Transmitting the force - hydraulic hose 
'The lorry is stuck - the pipe she's broken'. All too often hydraulic failure is simply a lack of standards in hose fittings. There are far too many components sold on price today that have no certified standard for comparisons or identification and trace-ability, so that when there is a failure no one is responsible. It's not just failure - leaking hydraulic hoses are an environmental curse and some companies will not allow a truck onto their premises if hydraulic leaks are detected in terms of ISO 14000 standards. More lost productivity!

Determining proper hose for a hydraulic application depends on size, temperature, application materials pressure, ends and delivery. And it's best to stick to brands that adhere to SAE standards - particularly the SAE J517 hydraulic hose standard. This also applies to the current SAE 100R series of hoses.

Hydraulic specifications must be matched in terms of pump speed / pressure ratings / hydraulic hose sizes. Mismatched pumps and piping can result in excessive suction where a pump takes away faster than hydraulic pipes allow causing pump cavitations. In addition, undersized piping builds up friction and heat with changes in hydraulic fluid viscosity index. 

Cross comments: "Hose is designed to handle fluid at a specified optimum pump rpm. Over-revving the truck engine to speed up the systems is damaging. Engine rev limiters must come into play as soon as a PTO and hydraulic system is engaged."
 

Cheap toggle switch Fitting the cheapest valve toggle switch is common practice. A switch should have safety protection – it should not be fitted without protection against accidental switch-on. This example is at least equipped with a red warning lamp to show it has been engaged.

Hydraulic fluid requires a high VI
Viscosity is the science of 'a fluid's resistance to flow' and is the key to the performance of any lubricated or hydraulic machinery. The viscosity of liquids decreases as temperature increases - this creates a temperature factor for indexing.

VI is viscosity index, a lubricating oil quality indicator and an arbitrary measure for the change of kinematic viscosity with temperature. Thus VI is a number usually between 0 and 200 and the temperature dependence measure of a specific oil viscosity. The VI scale was set up by the Society of Automotive Engineers (SAE). The temperatures chosen arbitrarily for reference are 100 °Fahrenheit (40 °C) and 210 °F (100 °C). The original scale only stretched between VI=0 (worst oil) and VI=100 (best oil) but since the conception of the scale better oils have also been produced, leading to VI's greater than 100.

Using SAE 30 mono-grade oil slows down hydraulic performance because it has a low VI of around 103. Compare this to an SAE 15W-40 multi-grade lubricant with a VI of 143 and it's clear that when wintery conditions are the ambient temperature start to a day's work, that hydraulics are not going to be productive with SAE 30. Cross points out that Mono-grade oils are not preferred for modern diesel engines, so it makes so much more sense to use standard multi-grade oil for an engine and the hydraulics fitted to a truck and avoid the chance of cross-pollution.
 

Hydraulic tip valve A hydraulic tip valve must synchronise with an entire system. Why fit the cheapest tipping valve when it should last as long as the truck? Does the valve incorporate performance characteristics that will match a variety of operating conditions?

Contamination is the enemy
Equipment surveys show that 90% of problems relating to downtime or premature failure are due to contaminants in hydraulics systems. It's all due to lack of training, not cleaning hydraulic components before stripping and pollutants entering during topping-up maintenance.

Where filters are not fitted to hydraulic systems, the lack of a R180 filter to protect a R15 000 to R20 000 hydraulic system is really penny-wise and pound foolish. Cross, however, goes further than one filter.

"While one filter is often only fitted to the return line, this is totally inadequate. It's best to have three filters. These three are fitted to the pressure line, return line and a bypass filter that runs continuously as soon as the truck engine is started to really keep hydraulic fluid clean. A pressure line filter is not negotiable on any system. In addition filters should achieve a minimum of Beta 10=100, or 10 micron at 99% efficiency," he says.

And what about the vital but overlooked hydraulic system breather? It has to accommodate the flow rate of air passing into the system as the cylinder is extended. "Breathers should offer a 10 micron or less efficiency," he advises.

A clogged filter will restrict the air flow into the reservoir as a cylinder is being extended, resulting in pump cavitation. Change the breather at regular intervals, or better still, use a breather with a contamination indicator. 

Hilton Cathcart, general manager at Jost Transport Equipment, has experienced that "diesel fuel and hydraulic oil stored in a single tank with a separating baffle results in cross pollution should the baffle crack or leak." Either way, a truck engine and hydraulic system will suffer damage.
 

Hilton Cathcart General Manager Jost Transport Equipment – “ Well-designed cylinders have a recommended operating speed – making this go up faster will damage sliding components.”

Operational problems & hydraulic failure
A badly designed tipper body with 900 corners at the floor and sides has a tendency to suffer sticking loads during the tip cycle - the product jams into the corners and will not discharge rapidly. Cathcart observes that "drivers then rock the truck via the driveline to try get a clean tip. The rocking motion of the truck with the hoist extended will damage the cylinder and seals."

Then there's the mysterious case of the Namibian-based tipper who sometimes refused to tip in the mornings. What was overlooked was the fact that the truck had been parked overnight in the open and loaded. An evening thunderstorm would wet the soil in the back to the extent that the sodden cargo now exceeded the hoist lift capacity. With morning sunshine, who would remember that the evening's downpour had drastically changed the payload in the body?

Take charge of the cause - not the effect
It's simply just not good enough to start replacing hydraulic system components without researching the reason for premature failure. Hydraulic failure 'on the road' is very costly to fix - and don't forget to add downtime and productivity losses to the final total.

The final word goes to Garnett Cross who has 'been-there-done-that' and has the T-shirt stained with hydraulic fluid. "The load determines the system, the pump flow rate determines the cylinder speed, the height to be lifted decides the cylinder stroke and length, further allowing selection of working pressure to match the equipment. And don't forget the correct pressure relief valve setting."

Sure, every reputable system has a safety margin but don't push Pascal's law too far. Hydraulic failure can be an expensive disaster - especially if a cylinder collapses. 

Pascal's law states – ‘increased pressure at any point in a confined fluid results in an equal increase at every other point in the container’. And Pascal's law allows forces to be multiplied.

Who was Pascal? Whenever you pressure your tyres – in kPa or kilo-Pascal – you reserve a memory moment for Blaise Pascal a 17th Century French mathematician. Pascal’s short life – 1623 to1662 – was one of pure genius. In addition to being a physicist, he was a religious philosopher and master of prose. He laid the foundation for the modern theory of probabilities and formulated what came to be known as Pascal's law of pressure