A Checklist for Sizing Hydraulic Filters

Typically flow velocities in hydraulic subsystems should be high enough to ensure the subsystems run smoothly and efficiently. In pump suction lines, flows should be travelling roughly 2-4 ft. per second (fps); in pressure lines, 10-25 fps is the range for fluid velocities; and for return lines, the figure is 5-10 fps.

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What is the working system pressure? The answer to this question will let designers choose a filter that will withstand that anticipated pressure. If the filter can&#;t and it collapses, it will likely lose its integrity, break down, damage the entire hydraulic subsystem and lead to a possible breakdown. To make matters worse, when a filter collapses, fragments of epoxy or filter media are injected into the flow stream and can damage downstream equipment.

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What is the fluid? Is it standard hydraulic fluid? Is it a petroleum-based product compatible with the equipment? These answers will let designers and operators know if the fluid is compatible with all system components as well as filters. If it is not, be prepared for serious system damage or destruction.

For example, if phosphate ester is the process fluid, polyester, nylon and stainless steel filter media are fine, but polypropylene is unacceptable. Standard hydraulic oils are compatible with filter media such as cellulose, polyester, polypropylene, stainless steel and most other types of media. The point is to ensure that the process fluid, filter media and seal material are all compatible. If there are any compatibility issues in any area, there will be leaks and major contamination problems.

What is the fluid&#;s viscosity? Standard hydraulic fluid has a viscosity of 150-200 SUS (Saybolt Universal Seconds, the time [in seconds] for 60 ml of oil to flow through a standard orifice at a given temperature). Any fluid with a higher viscosity is thicker than standard hydraulic fluid. Fluids with higher viscosities create higher pressure drops as they flow through the system, especially during cold weather start-ups. Designers and operators should be aware that allowable initial pressure drops vary widely from component to component and so they should know system requirements for pressure and ensure they will be met.

What type of pump is being used? Is it a piston, gear, vane or other type of pump? This is important to know when installing a new or replacement suction strainer. Suction strainers keep contaminants out of the pump, the most important component in a hydraulic subsystem. Different types of pumps sometimes require more protection. The chart below provides general guidelines for adequately protecting hydraulic pumps.

In this article, we explain the pros and cons of polyester and nylon mesh for screen printing.

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Like every other printing process where several parameters are to be critically controlled, screen printing, though a simpler process, also needs a strict check on various factors to achieve accurate results. This starts right from selecting a right mesh fabric, mesh size, frames, stretch pressure and many such variables depending upon the requirements of applications.

When it comes to screen fabric, there are two kinds available in the industry: nylon and polyester. While polyester is being widely used worldover, nylon is the choice of Indian screen printers for majority of applications.

The polyester mesh has several superior properties such as  resistance to solvent, high temperature, water, chemicals etc. Also, when acted upon by severe external pressure, its physical performance is stable and stretchability is low. However, as compared to nylon, polyester has poor wear resistance.

Thanks to its dimensional stability, polyester mesh is suitable for applications, where critical control of registration is required such as printing on printed circuit boards (PCB). The drawback of Nylon mesh is large stretchability and low tension retaining capacity. Therefore, after a period of time, the net stretch will reduce and make the screen plate flabby leading to decline in the level of precision. Therefore, it&#;s not suitable for printing on PCB.

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Screen printing being a wet process, moisture absorption is an important consideration. Nylon, for instance absorbs up to 5% moisture and loses 10-20% of its strength when wet and elongates 26-40% before breaking, while polyester elongates much lesser 19-23%.

Today, high-quality monofilament polyester, the modern mesh fibre is the most suitable mesh fabric. It is round and smooth with superior elongation and strength characteristics. It has no loss of strength when wet and, in fact, absorbs less than 0.8%moisture.

Nylon mesh, therefore, should only be used when printing on irregular surfaces or three-dimensional objects (such as bottles) that might require such properties.

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