Have Any Questions?

01902 491 118

Operating Hours

Mon - Fri 07:30-17:00

Email Us


Air Filter Servicing, Best Practises, Part 1 of 4.

The Filter Guys
  1. Don’t remove an air filter from its housing simply to inspect it.
  • Removing and replacing the same filter during air filter servicing can do more harm than good.
  • Ridges of dirt on the gasket sealing surface can drop on the clean filter side when the gasket is released.


  1. Never hit a filter to try cleaning it.
  • Rapping hard enough to knock off dust damages the filter and can place your engine at risk for dust ingestion.
  • Deeply embedded dirt is never released by tapping.
  • It is always safer to keep operating until you can change to a new filter than to try and tap out the dirt.


  1. Never operate a system with only a safety filter in place.
  • Safety or secondary filters used alone will let harmful contaminant enter the engine.
  • Safety or secondary air filters are designed to compliment the primary filtration or provide protection during primary filtration service.


  1. For longer service between filter changes, consider upgrading to an extended service filter such as Donaldson Endurance™ air filters. Then service the filter by restriction only.

  1. Ideally, service your air filter by restriction measurement or follow

your regular maintenance schedule.

  • If you don’t trust your current filter service indicator, getting a new one is a good idea.
  • Restriction indicators, mounted on the air cleaner system are recommended for keeping an eye on restriction levels and indicating when servicing is due.
  • For testing of initial restriction and confirming remaining filter life, we recommend the greater accuracy of a clock-type restriction gauge or water manometer.

Hydraulic Filtration Pressure Drop

The Filter Guys

Hydraulic Filtration Pressure Drop


The difference between the inlet pressure and the outlet pressure is called pressure drop or differential pressure. It’s symbolized by ∆P. ∆P is an irrecoverable loss of total pressure caused by the filter, and is mostly due to frictional drag on the fibers in the media.

Differential drop drop may increase as the particulate rating or efficiency of the filter (as expressed by its beta ratio) gets better. ∆P also increases as the filter is being loaded with contaminant.


Four Major Factors Contribute to Pressure Drop


  1. Filter Media.


Media is, of course, the main factor influencing pressure drop; indeed, it causes pressure drop. That’s why having a low-friction, high-flowing media is so important. The natural cellulose or paper fibers (shown at left) typically used in filtration are large, rough, and as irregular as nature made them.

Donaldson developed a synthetic media with smooth, rounded fibers, consistently shaped so that we can control the fiber size and distribution pattern throughout the media mat, and still allow the smoothest, least inhibited fluid flow. Our synthetic media is named Synteq™.

Synteq fibers offer the least amount of resistance to fluid passing through the media. Consistency of fiber shape allows the maximum amount of contaminantcatching surface area and specific pore size control. The result is media with predictable filtration efficiencies at removing specified contaminants (i.g., 4 µm) and maximum dirt holding capacity.

Natural cellulose fibers are larger than synthetic fibers and jagged in shape, so controlling size of the pores in the media mat is difficult and there is less open volume. In most applications this results in higher ∆P as compared to synthetic filters. Higher beta ratings mean there are smaller pores in the media; smaller media pores cause more flow resistance, in turn causing higher pressure drop.


  1. Dirt.


Contaminant As dirt gets caught in the media, it eventually begins to build up and fill the pore openings. As the pore openings shrink, the differential pressure (pressure drop) increases. This is called restriction. This photo from our scanning electron microscope shows actual dirt particles building up in the media pores.

Excessive dirt in the media can cause dirt migration or even filter failure. Dirt migration occurs when the restriction is so great that the differential pressure pushes dirt deeper into the media and, eventually, through the media and back into the system. Filter failure occurs when the restriction becomes so high that the filter cartridge collapses (outside-in flow) or bursts (inside-out flow) to relieve the upstream pressure.

To avoid such catastrophe, use of a filter service indicator is recommended. It measures the pressure drop across the filter, then signals when the filter is ‘full’ and needs to be changed.


  1. Flow


Higher flows create higher pressure drop. With fast moving fluid, there will be more friction causing higher pressure drop across the media.


  1. Fluid Viscosity


Measured in centistokes (cSt) or Saybolt Seconds Universal (SSU or SUS), fluid viscosity is the resistance of a fluid to flow. As fluid viscosity increases, the cSt rating increases. Higher fluid viscosities also mean higher pressure drop because the thicker oil has a tougher time passing through the layer of media fibers. Cold start fluid is a good example of highly viscous fluid. See chart below.

Filter media, amount of contamination, the flow rate, and fluid viscosity are all factors in the importance of sizing the filter for the system requirements. Filters that are too small won’t be able to handle the system flow rate and will create excessive pressure drop from the start. The results could be filter operation in the bypass mode, filter failure, component malfunction, or catastrophic system failures. Filters that are too large for the system can be too costly. Oversized filters require more system oil and higher cost replacement filters. Optimal sizing is best.

Types of Hydraulic Fluid

The Filter Guys


Types of Hydraulic Fluid

There are many kinds of fluids used for power, but they can basically be called petroleum-based fluids, biodegradable fluids, and fire-resistant fluids. A brief description of some of the types in each category are listed below; for details on these or others, consult your filter supplier or refer to a reputable manual on hydraulics, such as the Lightning Reference Handbook, published by Berendsen Fluid Power, Whittier, CA 90601.


Petroleum Based (Hydrocarbon)

These are the most commonly used fluids in hydraulic systems. Their major advantages are low cost, good lubricity, relatively low/non-toxicity, and common availability. This type of fluid is not just plain oil; rather, it is a special formulation with additives that make it suitable for hydraulic systems. Mostly, the additives inhibit or prevent rust, oxidation, foam and wear.



  • Straight oils: same as petroleum-based oil but without the additives.
  • Automatic transmission fluids (ATF): excellent low temp viscosity and very high VI.
  • Military hydraulic fluids (ie: MIL-H-5606 and MIL-H-83282): also called ‘red oil’ because of the colour. Low viscosity, good for cold temp operations, but may have to be modified for pumps.


Fire Resistant Fluids.

There are two types of fire-resistant fluids commonly used in hydraulic applications: Phosphate Esters and High Water Based Fluids (HWBF). Although generally not as viscous at cold temperatures as petroleum-based fluids, they are fire resistant due to their high content of non-combustible material. Very useful in overcoming the likelihood of fire caused by a broken hydraulic line spraying petroleum fluid into a pit of molten metal, onto a hot manifold, into a heat treating furnace, or other ignition source.


Some types of HWBF:

  • Oil-in-water emulsions (HFA): typically 95% water and 5% oil, with the oil droplets dispersed throughout the water. Provide some fire resistance, but due to oil content, other fluids are superior.
  • Water-in-oil emulsions (invert emulsion HFB): typically 40% water and 60% oil, with the water dispersed in the oil. Provide some fire resistance, but due to oil content, other fluids are superior.
  • Water-glycol (HFC): typically 40% water and 60% glycol. Excellent fire resistance. Since glycol is an antifreeze, water-glycol can be used at lower temps. NOTE: HWBF may require reduced pressure rating of pumps and other components.


HFD Fluids.

The HFD group is a classification given to several different types of synthetic products that do not contain petroleum oil or water. Phosphate ester fluids were the first HFD fluids and are the most fire resistant within the HFD family. Not as popular today, their use declined due to poor environmental performance, limited compatibility, and high cost.

Certain phosphate esters have very high auto-ignition temperatures and are still used in specific applications, such as aircraft and power generation.


A common brand is known as Sydrol® (registered trademark of Solution, Inc.). Skydrol requires EPR seal for chemical compatibility. Today most phosphate esters have been replaced by polyol esters. Based on organic esters, polyol esters are the most common HFD fluids used today. They offer good inherent fire resistance, good compatibility with system materials, excellent hydraulic fluid performance, and easy conversion from petroleum oil. In addition, the organic nature of these fluids gives them good environmental performance in biodegradability and aquatic toxicity. Another type of synthetic, fire resistant fluids have been formulated for certain niche markets.


Water free polyalkylene glycols (PAGs) feature extended fluid life and good environmental performance. Technically an HFD fluid, PAGs (also known as polyalphaolefins (PAOs) are more often used for their biodegradability and overall environmental friendliness. This group also contains the synthetic silicone (siloxane) oils, known for their anti-foaming properties.



With increasing concern about the environmental impact of hydraulic system leaks and spills, biodegradable fluids are receiving expanded usage, particularly in Europe. There are two types of common biodegradable hydraulic fluids: 1) vegetable-based oils, such as sunflower or rapeseed oils, and 2) synthetic oils like diesters, etc. Generally, systems using biodegradable fluids are derated for maximum and minimum temperatures. Users who replace standard hydraulic oils with biodegradable oils must check with filtration component manufacturers to confirm that the fluid and components are compatible.



The Filter Guys

Schupp filters are now available from Sterling Filtration Ltd. Schupp are a worldwide brand offering 1000’s of aftermarket air, lube oil, fuel and hydraulic filters for many common on and off highway applications. We can offer competitively priced Schupp original filters, what ever your current Schupp original requirements may be.

If you require Schupp filter replacement elements please give us a call today.


The Filter Guys

Wix filters are now available from Sterling Filtration Ltd. Wix are a worldwide brand offering 1000’s of aftermarket air, lube oil, fuel and hydraulic filters for many common on and off highway applications. We can offer competitively priced Wix original filters, what ever your current Wix original requirements may be.

If you require WAM filter replacement elements please give us a call today.

Compatibility of Filter Media with Hydraulic Fluids

The Filter Guys

Compatibility of Filter Media.

Filter media can be divided into two broad categories, natural fibres (usually cellulose) and synthetic or man made fibres (usually fibre glass or woven wire mesh), although many different variations and types of media do exist, many filters fall into one of these two categories.

Fluid to be Filtered Recommended Media
Petroleum-based Cellulose, Glass fibre
Phosphate Ester Glass Fibre
Diester Glass Fibre
Water Glycol Glass Fibre
Water-Oil Emulsion Glass Fibre
Biodegradable Fluid HWCF* Glass Fibre
Course Filtration Wire Mesh

* High water content filtration

Notes on seals:

Filters with seals made of BunaN are appropriate for most applications involving petroleum oil and some high water content fluids. Filters with seals made of Viton® of Fluorel® (both fluoroelastomers) are required when using diesters, phosphate ester fluids.

(Viton and Fluorel are registered trademarks of DuPont Dow Elastomers and 3M)

Don’t Retighten a Lube or Oil Filter Once it has Been Installed.

The Filter Guys

To avoid cross-threading DO NOT OVER TIGHTEN OR RETIGHTEN a filter. Follow These Proper Steps for Installation of Engine Lube Spin-on Filters

The spin-on engine lube filter is a commonly serviced component on your vehicle. To insure normal oil filter service life and to prevent oil leakage or possible internal engine damage, the following procedure should be followed when replacing spin-on engine lube filters.

Before Installation:

  • Unscrew and remove the old filter.
  • Remove the old filter gasket if still attached to the base.
  • Wipe the filter mounting base or head with a clean cloth.

To Install a Spin-on Lube Filter:

1. If manufacturer recommends, fill new filter with oil on the dirty side. Do not pour the oil down the clean oil centre tube.

2. Apply a thin film of clean motor oil to the new filter gasket. Do not use grease.

3. To avoid cross-threading, line up the filter threads to the mounting base/head carefully. Screw the new filter on until the gasket contacts the base.

4. Follow the instructions on the filter for final tightening torque specification. DO NOT OVER TIGHTEN.

5. Check the gasket of the new filter to be sure it is properly seated in the groove.

Note that not all filters require the use of tools for changing.

After Installation:

6. Be sure the oil reaches the full level on the dipstick.

7. Start engine and check for leaks around the oil filter and drain plug. Correct the source of leaks if any are observed.

8. Shut the engine off and recheck the oil level. Add oil if needed.

Always dispose of old oil and used filters responsibly.

Don’t Throw Away a Good Filter Just Because it Might Look Dirty.

The Filter Guys

Don’t Throw Away a Good Filter Just Because it Might Look Dirty.

Although this air filter may look “dirty” – It can go plenty more miles. Installation of a restriction indicator can save you money and time.Why Service By Restriction?

Proper air cleaner servicing will result in maximum engine protection against the ravages of dust. Proper servicing can also save you time and money by increasing filter life and dust cleaning efficiency. By using proper filter restriction measurement tools you will use the full life of the filter at maximum efficiency. Don’t be fooled by filter appearance: it should look dirty.

The only way to determine when a filter is plugged or plugging is to measure the restriction on the system with the engine working at max airflow.

Two of the most common air cleaner servicing problems are:

1. Over-servicing: the least efficient time in the life of the filter is when it is new. Filter elements increase in efficiency as dust builds up on the media.

2. Improper servicing: your engine is highly vulnerable to abrasive dust contaminants during the servicing process when the filter is removed from the housing. A leading cause of engine damage is due to careless servicing procedures.

Watch Out for Old Compression Gaskets.

The Filter Guys

Watch Out for Old Compression Gaskets. When changing any filter that has a gasket – use caution as old gaskets may stick.

A compression seal is a means of preventing migration of liquids, gases or solid contaminates across a joint or opening in an assembly or housing. A seal not only prevents the escape of fluid from inside and foreign material from entering the system from outside, but it must provide for easy installation and removal. A new gasket is critical for proper filter function.


  • Remove used gaskets and clean the sealing area thoroughly
  • Always use a new gasket with a replacement filter
  • Over-tightening the filter may damage the head
  • Dispose of used filters properly

Worried About Water in Your Air Intake System?

The Filter Guys

Worried About Water in Your Air Intake System? Sometimes you can’t help operating equipment in extreme moisture environments, but it’s good to know a few things to help keep your air intake system running at top efficiency.

Typical Symptoms of Water Ingestion:

  • High Restriction indications.
  • Mud caked in the VacValve
  • Wet, wavy air filter media
  • System rust, corrosion and/or water damage
  • Moisture-related environmental problems such as icing


Simple Tips to Keep Water Out of Your System:

  • Check and clear the VacValve daily
  • Make sure the air cleaner cover and filter are installed properly
  • Inspect air intake system for any leaks

Caution: A water-soaked air filter will occasionally lock-up a restriction indicator!

A restriction indicator’s ‘lock-up” restriction level is generally marked on the indicator itself. To check an indicator, remove it, wipe the base clean, then apply a small amount of vacuum. If the indicator locks up, it should still be okay to reuse this way. If not be sure to replace the indicator. Air filters, complete air cleaner assemblies and associated spares and parts are available from Sterling Filtration Ltd.

1 2 3 4 5 10