A failed filter will wreak havoc in a hydraulic system. Particles that aren’t trapped in the filter media will cause wear, damage seals and block small orifices like those in servo valves. Without effective filters hydraulic equipment will fail prematurely, needing expensive repairs and causing costly downtime. That’s why every hydraulic pressure system is designed with filters.

Two essential but often overlooked aspects of a hydraulic system filter are the inner support tube and the outer protective cage. These provide the structural integrity needed to prevent failure but at the same time must let the hydraulic fluid flow freely. These two objectives may seem incompatible, but that’s where perforated tubes get involved.


Specialists in Perforated Tubes

We manufacture perforated tubes for use in a wide variety of products and industries. One of those applications is hydraulic pressure filtration systems. We can produce tubes in many different metals, from very small to very large diameters, and with an almost unlimited variety of perforation patterns. It’s these capabilities that make our perforated tubes an essential component of many different filters.


Hydraulic Fluid Filtration

Hydraulic systems work by putting fluid under pressure. Transferring force from one place to another, this operates cylinders and valves in equipment ranging from construction and agricultural machines to precision machinery and test systems. One thing these systems have in common is that they need very clean fluid.

Particulate contamination in hydraulic fluid accelerates wear and brings about premature failure. Contamination by other fluids, especially water, can cause corrosion and cavitation. Water is a particular problem as it can boil and become a gas. Unlike liquids, gasses are compressible and reduce the fluid’s ability to transmit force.

Most hydraulic pressure systems have at least two filters. There’s one at the pump inlet protecting it from larger particles and a second filter on the line returning fluid to the reservoir. Inlet filters are generally designed to trap particles larger than 75 microns, (technically, this makes them strainers,) while return filters target much smaller particles.

Filters typically have bypass systems that let fluid keep flowing should the filter become blocked. These protect against fluid starvation but let unfiltered fluid move through the system.

Hydraulic systems that use servo valves often incorporate additional pressure line filters. These safeguard the small orifices against blockage. Such filters usually don’t include bypasses because fluid starvation is preferable to a blockage.

Many hydraulic systems also incorporate offline filtration in the reservoir. These can provide a continuous fluid cleaning mechanism even when the main pump isn’t running.


Filter Construction

Filters trap particles while allowing fluid through. To do this the filter material is full of small holes or pores. Common filter materials include paper, woven wire and fiberglass. The material is engineered to provide filtration down to a target particle size. However, smaller pores impose more resistance to flow and increase pressure drop.

Filter manufacturers address this by increasing the filter area. This is why so many filters have a pleated construction wrapped around an inner support tube.

This support is almost always made from perforated tube. Without the tube the filter material would quickly crush under pressure from the fluid.

Surrounding the filter material is a protective outer cage. While some filters use wire or plastic a perforated metal tube is a more stable and dependable solution.


Design Considerations for Hydraulic Pressure System Perforated Tubes

Many factors are considered during the design process, but from a product function perspective the main ones are:

  • Perforation pattern
  • Material
  • Material thickness

Perforation pattern determines how fluid flows to and from the filter material. It should optimize flow and pressure drop while ensuring load on the material is evenly distributed. Achieving this entails engineering hole shape and arrangement as well as the “open area.” (“Open area” is the ratio of total hole area to metal surface.)

A pattern of offset circular holes is often the starting point but strength, resulting pressure drop and fluid distribution must all be taken into account. For example, larger diameter, densely-packed holes reduce pressure drop by increasing open area but may result in a weak tube and potentially damaging flow velocities.

In some applications a square or hexagonal hole pattern may offer advantages. It has also been suggested that oval holes provide better flow than those that are circular. On the outer cover hole pattern also offers potential for branding or other design differentiation.

Required strength, target weight and characteristics of the particular hydraulic fluid all influence material selection. Stainless steel is usually the first choice but brass, copper, aluminum or mild steel may be better in particular instances. In some cases magnetic properties and electrical conductivity must also be considered.

Thickness is usually addressed in conjunction with material selection. Thicker material is stronger but will likely add weight and perhaps bulk to the filter. A stronger but more expensive material might reduce overall size and weight and result in a filter more attractive to the end user.


Dependable Performance

If the inner support tube or outer cage of a filter fails, contaminated fluid can flow through a hydraulic system undetected. Conversely, a tube or cage might be strong enough but without sufficient attention to design, could impose excessive pressure drop or allow the fluid flow to damage the filter.

In either scenario the result is costly downtime and expensive repairs. You can avoid these problems and their subsequent impact on your reputation by using only high quality perforated tubes in your filters. Call or email to learn more.

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