Seven steps to hydraulic cleanliness

01 December, 2015

Within a hydraulic system, contamination breeds contamination. Initial damage, including corroded surfaces or eroded seals, sets off a chain reaction of wear meaning gaps grow larger, leaks increase in size and metal-to-metal contact increases, further contaminating oil and leading to operating inefficiencies and control inaccuracies. While contamination can occur as a result of wear to internal parts or through water or air ingress, a common cause is insufficient filtration or cleanliness of lubricants, meaning the contamination is being introduced to a previously acceptable system. As today’s systems are smaller and more powerful, so the demands of cleanliness become much higher and the size of particle which can disrupt and damage hydraulics becomes imperceptible to the human eye.

On average, the human limit of vision is a particle of 40µ - put into perspective, a grain of salt is typically 100µ while a hair is around 70µ. Typical component clearances however range from 25µ down to a microscopic 0.5µ as follows:

• Gear pump – 0.5-5µ

• Vane cell pump – 0.5-5µ

• Piston pump – 0.5-1µ

• Control valve – 5-25µ

• Servo valve – 5-8µ

Most new oil is supplied at a filtered level of 20µ, which for most hydraulic systems will be insufficient as any oversized particles can cause blockages and negatively affect pressure in the system.

ISO 4406:1999 is the standard by which the level of acceptable contamination by solid particles can be set. Also known as the ISO cleanliness code, it helps to set the maximum level of particles present per millilitre of oil, at three different sizes – 4µ, 6µ and 14µ. This maximum level then correlates to a range number, giving each component a three-number code (for example 16/14/11 for a servo control valve – and once this code is known, sampling or live monitoring devices can be set to this level to alert engineers of any deviation from the allowed limits. Corrective action can then be taken in advance of catastrophic failure and costly unplanned downtime. But what factors need to be taken into account when determining the ISO code?

The ‘seven steps’ to achieving the correct levels of hydraulic cleanliness were originally introduced by the manufacturers of filters used within hydraulic systems as a means of selecting the correct filter media and depth, and can also be used to determine the filtering requirements of oil before it enters a hydraulic system.