How to Deglaze Hydraulic Cylinders

Is it necessary to deglaze the cylinder tube and how do you do it?

While it’s not always necessary, it’s usually a good idea. After long periods in service, the internal surface of cylinder tubes can actually become too smooth. As a result, the replacement seals may leak.

Deglazing/deburring also removes spot rust and other minor surface imperfections – which is essential for optimum seal life.

The tool that’s most often used for this job is a Flex-Hone. The correct technique for using one is shown in the video “How to Use a Flex-Hone”:

http://www.brushresearch.com/videos.php

If your going to try to repair your hydraulic cylinders, it’s well worth three minutes of your time.

Craig Cook

Cost of Air Leaks

Air leaks are much easier to ignore than oil leaks because they don’t draw attention to themselves in the same way.

You don’t need to worry yourself with clean-up and disposal costs. Contaminant ingression is possible, but is generally not a major concern.

And unless the leak is significant, safety is not usually a big issue either. So that leaves make-up fluid (air).

While air is free – clean, dry compressed air is NOT.

In considering the cost of make-up air for a pneumatic system the following need to be considered:

Depreciation (wear and tear) of the compressor; Conditioning costs – filtration, drying and lubrication; and Energy cost of compression.

The ideal leakage rate is of course zero, but when calculating the free-air delivery (FAD) required by a pneumatic system a rule of thumb is to allow for leakage of 10% of total flow rate.

Consider a 10 cubic meter/minute system leaking 10% or 1 cubic meter/minute. The power required to compress 1 cubic meter (35.3 cubic feet) of air per minute to a pressure of 6 bar (90 PSI) is approximately 5.2 kilowatts. At an electricity cost of $0.10 per kilowatt-hour this leakage is costing over 50 cents per hour in electricity consumption alone.

In a 24/7/365 operation that amounts to $4500 per year!

While a leakage rate of 10% of flow rate may sound high and would be unsustainable in a hydraulic system, air leakage rates as high as 25% are not unheard of even in apparently well maintained pneumatic systems!

As demonstrated by the above example, the annual cost of air leaks in pneumatic systems can be significant in power consumption alone.

Craig Cook

Monitoring Filter Condition

Let’s change gears and discuss the benefits of monitoring the condition of your hydraulic filter elements.

Warning of filter-bypass is typically afforded by visual or electric clogging-indicators. These devices indicate when pressure drop (delta P) across the element is approaching the opening pressure of the bypass valve (where fitted).

In the case of a return filter for example, if the bypass opens at a delta P of 3 Bar, the clogging indicator will typically switch at 2 Bar.

Replacing standard clogging-indicators with differential pressure gauges or transducers enables continuous monitoring of the filter element.

Think of it in terms of the engine coolant temperature on your car or truck. Would you prefer to have a gauge on the dashboard that allows you to monitor coolant temperature continuously or a light which only glow when the engine has already overheated?

Similarly, continuous monitoring of the filter elements in a hydraulic system can provide valuable clues to the performance of the filter and the condition of the system.

This allows trending of fluid cleanliness against filter pressure-drop, which may be used to optimize oil sample and filter change intervals. For example, the optimal change for a return filter in a particular system could be higher or lower than switching pressure of the clogging indicator.

Continuous monitoring may also provide early warning of component failures and element rupture. For example, if the delta P across a pressure filter suddenly increased from 1 to 3 Bar (all other things equal), this could be an indication of an imminent failure of a component upstream.

Similarly, a sudden decrease in delta P could indicate a rupture in the element – something a clogging indicator will never alert you to.

Craig Cook

Back to the Basics: Hydraulic Pumps

I’m going back to the basics with a review of hydraulic pumps. Let’s dive right in!

Positive-displacement pumps, also called hydrostatic pumps, are used in fluid power motion control applications. They have a very small clearance between rotating and stationary parts. These pumps deliver a specific amount of fluid to the system for each revolution.

Positive-displacement pumps can be further divided into two categories: Fixed- and variable-displacement.

Fixed-delivery pumps provide a single, specific volume displacement per revolution.

In variable-displacement pumps, displacement per cycle can be varied from zero to maximum volumetric capacity. Some of the more widely used types of positive-displacement pumps are gear, piston, and vane types.

Gear pumps can be either internal or external styles. External gear pumps are one of the most popular types used in modern hydraulic systems. Gear pumps produce flow by using the teeth of two meshing gears to move the fluid. Their simple construction ensures limited purchase costs and servicing. Gear pumps work under heavy operating conditions and transmit high amounts of hydraulic power. They feature decent hydraulic, mechanical and volumetric efficiency, compact dimensions, and low weight/power ratio. This balance of efficiency and economy make external gear pumps a popular choice for auxiliary systems in a number of different machines.

External gear pumps can be equipped with straight spur (the most common type), helical, or herringbone gears. In operation, the drive gear and driven gear rotate, creating a partial vacuum at the pump inlet (where gear teeth unmesh) that draws fluid into gear teeth. Gear teeth mesh at the outlet, forcing fluid out of the pump.

Internal gear pumps contain one internal and one external gear. They pump fluid in the same manner as external spur gear pumps. In the basic design, the internal gear, which drives the outer gear, has one tooth less than the outer gear. As they mesh, the teeth create sliding seal points. Because their transition zone from low to high pressure (the area over the crescent) is relatively long, internal gear pumps can offer lower noise levels than some other types of pumps.

Gears are made of special steel and are often case hardened and quench hardened. Then gears are ground and fine finished. Proper tooth profile design and geometric proportions can reduce pulsation and noise levels during pump operation.

Piston pumps supply high flows at high rpm. Two types of piston pumps, axial- and radial-piston, are manufactured in both fixed- and variable-displacement versions. Axial-piston pumps contain one or more pistons that convert rotary shaft motion into axial reciprocating motion. An angled cam (or wobble plate) rotates, causing pistons to reciprocate and take fluid in as they move toward the thin part of the plate. Fluid is expelled as pistons approach the thick end. In one version, the bent-axis design, both pistons and shaft rotate, making a wobble plate unnecessary. Bent-axis pumps use the drive shaft to rotate pistons.

With the longer sealing paths along the piston walls, piston pump efficiencies tend to be higher than other types of pumps. In addition, variable-displacement pumps can provide savings by only providing the pumping necessary for the function, saving additional energy and costs.

Radial-piston pumps (fixed-displacement) are used especially for high pressure and relatively small flows. Pressures of up to 5000 psi are common. Variable-displacement is not possible, but sometimes the pump is designed in such a way that the plungers can be switched off one by one, so that a sort of variable-displacement pump is obtained.

Radial-piston pumps are characterized by a radial piston arrangement within a cylinder block. As pistons reciprocate, they convert rotary shaft motion into radial motion. One version has cylindrical pistons, while another uses ball-shaped pistons. Another classification refers to porting: check-valve radial-piston pumps use a rotating cam to reciprocate pistons; pintle-valve pumps have a rotating cylinder block, and piston heads contact an eccentric stationary reaction ring.

Rotary vane pumps (fixed and simple adjustable displacement) generally have higher efficiencies and lower noise levels than gear pumps. They can be used for mid pressures of 2500 psi and modern units can exceed 4500 psi in continuous operation.

Some types of vane pumps can change the center of the vane body, so that a simple adjustable pump is obtained. These adjustable vane pumps are in general constant pressure or constant power pumps: the displacement is increased until the required pressure or power is reached and subsequently the displacement or swept volume is decreased until equilibrium is reached.

A critical element in vane pump design is how the vanes are pushed into contact with the pump housing, and how the vane tips are machined at this very point. Several type of “lip” designs are used, and the main objective is to provide a tight seal between the inside of the housing and the vane, and at the same time to minimize wear and metal-to-metal contact. Forcing the vane out of the rotating center and towards the pump housing is accomplished using spring-loaded vanes, or more traditionally, vanes loaded hydrodynamically (via the pressurized system fluid).

Considerations when selecting a hydraulic pump

Pressure
Angle
Related torque
Weight
Mounting configuration

Craig Cook

Material Types For Seals

After last week’s post, I have gotten questions about material types for seals.

The four most commonly used materials for sealing applications are polyurethane (PU), acrylonitrile-butadiene-rubber (NBR), fluoro-rubber (FKM), and polytetrafluoroethylene (PTFE).

Polyurethane (PU)

Polyurethane is an organic material of high molecular weight whose chemical composition is characterized by a large number of urethane groups. Urethanes belong to the thermoplatic elastomers (TPE) family and close the gap between thermoplastic and elastomeric materials regarding hardness, deforming behavior and consistency. Within certain temperature limits, polyurethane possesses the elastic characteristics of rubber combined with the advantages of a rigid plastic.

The composition of the material is determined by three components: polyol; diisocyanate; and a chain extender. The type and amount of these materials used, and the reaction conditions, are decisive in determining the properties of the resulting polyurethane material. In general, polyurethanes possesses the following properties:

High mechanical strength
Very high tensile
Good abrasion resistance
Good flexibility
Modulus of elasticity which can be varied within wide limits
Wide range of hardness values, while retaining good elasticity
Very good resistance to ozone and oxygen
Outstanding resistance to abrasion and tear
Good resistance to oil and gasoline

Temperature range for use: -30° to 80° C; high performance types (compounds) up to 110° C in mineral oils (long term exposure temperature).

Acrylonitrile-Butadiene-Rubber (NBR)

NBR is a polymer of butadiene and acrylonitrile. The acrylonitrile (ACN) component affects the following properties of the NBR:

Elasticity
Cold flexibility
Gas permeability
Compression set
Swelling resistance in mineral oils, greases and fuels

A NBR material with low ACN content has very good cold flexibility (down to approximately -45° C) and moderate resistance to oil and fuel. In contrast, a material with very high ACN content with optimum resistance to oil and fuels, may have a cold temperature flexibility only down to -3° C. With rising ACN content, the elasticity and the gas permeability decrease and the compression set becomes worse.

NBR provides:

Good resistance to swelling in aliphatic hydrocarbons, greases, fire retardant hydraulic fluids of Groups HFA, HFB and HFC, vegetable and animal oils and greases, light heating oil and diesel fuel
Good resistance to hot water at temperatures up to 100° C (sanitary fittings), inorganic acids and bases at concentrations and temperatures which are not too high
Moderate resistance to swelling in fuels having a high content of aromatics (super grades of fuel)
High swell in aromatic hydrocarbons, chlorinated hydrocarbons, flame retardant hydraulic fluids of the Group HFD, esters, polar solvents and brake fluids based on glycol

Temperature range for use (depending on the composition of the blend): -40° and 100° C and for short periods up to 130° C (the material hardens at higher temperatures). For special blends, the cold flexibility extends down to -55° C.

Fluoro-Rubber (FKM)

Copolymers, terpolymers or tetrapolymers with various compositions and with fluorine contents from 65% to 71%, which have varying resistance to surrounding media and varying cold flexibility can be made by polymerization of vinylidne fluoride (VF) and variable amounts of hexafluoropropylene (HFP), tetrafluoroethylene (TFE), 1-hydropentafluoropropylene (HFPE) and perfluoro (methylvinylether) (FMVE). Cross-linking is achieved either with diamines and bisphenols or with organic peroxides.

FKM provides:

Outstanding resistance to high heat
Excellent resistance to oil, gasoline, hydraulic fluid and hydrocarbon solvents
Good flame retardant
Low permeability to gases
High swell in polar solvents and ketones, fire retardant hydraulic fluids of the Skydrol type, and brake fluid

Newly developed materials (cross-lined by peroxides) have good resistance to media, which can only be tolerated to a small extent, if at all, by conventional FKM.

Temperature range for use: approximately -20° to 200° C (for short periods to 230° C). Special grades: -50° to 200° C.

Polytetrafluoroethylene (PTFE)

PTFE is a polymer of tetrafluoroethylene. This non-elastic material is characterized by a series of outstanding properties:

Slippery surface that repels most media
PTFE is non-toxic at working temperatures up to 200° C
Very low coefficient of friction against most opposing surfaces made of other materials; stiction and friction are almost the same
Excellent electrical insulating properties (almost independent of frequency, temperature and weathering effects)
Chemical resistance that exceeds that of all other thermoplastics and elastomers
Liquid alkali metals and a few fluorine compounds attack PTFE at higher temperatures

The temperature tolerance is between -200° and 260° C; PTFE has some elasticity even at extremely low temperatures and therefore is used in many extreme cold temperature applications.

Specifying seals

Sealing systems use a combination of specific seals that work together to offer superior performance. High-pressure rod end sealing systems normally consist of four elements: the wiper, the rod seal (or main seal), the buffer seal (or secondary seal), and the wear (or guide) bands. The piston sealing system consists of a main seal combined with wear bands.

Craig Cook

Simple Explanation of Seals

There are numerous sealing products, technologies, and techniques to keep oil in and contaminants out of the hydraulic system, each providing unique benefits. Further, in some harsh applications, the features of individual seals do not fulfill the demanding requirements for the equipment and a sealing system is required.

Here are the basics of seal designs:

Wipers:

Provide aggressive wiping force to prevent mud, water, dirt and other contamination
Allow lubricating oil film to return to system on inward stroke
Protect main sealing elements, thus increasing life of seals
Are often made from Polyurethane, which offers high abrasion resistance
Are often used as a linkage pin grease seal

Rod seals:

Contain system fluid from escaping to atmosphere
Must provide sealing function at low and high pressure
Require excellent extrusion and wear resistance
Should provide good pump-back capability for lubricating oil film
Often must withstand up to 6000 psi

Buffer seals:

Must withstand high pressure exposure
Protect the rod seal against pressure spikes
Pressure relieving capability prevents pressure build-up between seals
Increase rod seal life
Allow for wider extrusion gaps
Require high wear resistance

Wear bands:

Prevent contact between metal parts in the cylinder
Center rod and piston from housing elements
Increase seal life

Craig Cook

Could have avoided 4 hours of downtime…

Check out this recent troubleshooting situation by one of our guys:

The machine in question had a complex hydraulic system, the heart of which comprised two engines driving ten hydraulic pumps. Six of the pumps were variable displacement and four of these had electronic horsepower control.

The symptoms of the problem were slow cycle times in combination with lug-down of the engines (loss of engine rpm). The machine had just been fitted with a new set of pumps.

The diagnosis of the mechanic in charge was that the hydraulic system was tuned above the power curve of the engines, that is the hydraulics were demanding more power than the engines could produce, resulting in lug-down and therefore, slow cycle times.

The other possible explanation of course, was that the engines were not producing their rated horsepower.

Due to the complexity of the hydraulic system, he knew that it would take around four hours to run a complete system check and tune-up. So in order to eliminate the easy things first, when he arrived on site he inquired about the condition of the engines and their service history.

The mechanic in charge not only assured him that the engines were in top shape, he was adamant that this was a “hydraulic” problem.

Four hours later, after running a complete check of the hydraulic system without finding anything significant, he was not totally surprised that the problem remained unchanged.

After a lengthy discussion, he managed to convince the mechanic to change the fuel filters and air cleaner elements on both engines.

This fixed the problem. It turned out that a bad batch of fuel had caused premature clogging of the engine fuel filters, which were preventing the engines from developing their rated horsepower.

Had the relatively simple task of changing the engine fuel filters been carried out when the problem was first noticed, an expensive service call and four hours of downtime could have been avoided.

ALWAYS check and eliminate the easy things FIRST.

Craig Cook

Cheat Sheets to Make Your Job Easier!

Do you use our cheat sheets for fittings and components? The cheat sheets are ‘to scale’ so you can use for quick reference when you are not sure the size you need.

Many people print them off, laminate them, and keep them on the wall in their maintenance or machine shop for reference.

Click here for all the cheat sheets.

We have charts for:

NPTF/Pipe Flareless

BSPT NPSM

Metric Metric Bite Type

Kobelco SAE/Boss

Brass Straight Flat Face

Instrument. Komatsu

JIC Bump Tube

BSPP NPSH

Universal NST

JIS

Craig Cook

Tightening Taper-Thread Connections

When it comes to tightening taper-thread connections, you must exercise a degree of caution.

Not enough torque and the joint will leak.

But excessive torque can result in broken housings on pumps and valves – due to the force developed by the wedge action of the tapered adaptor.

This Parker document, provides a guide for torquing tapered-thread adaptors – based on the ‘turns from finger tight’ (TFFT) method:

tapered threads guide

And when it comes to taper-threads, this is about as scientific as it gets.

Craig Cook

Stopping the “Water Hammer” in Your Hydraulic System

In last week’s email, we talked about decompression and how to control it.

Let’s wrap up the topic of noise by talking about a phenomenon known as ‘water hammer’.

Water hammer is the term used to describe the effect that occurs when the velocity of the fluid moving through a pipe suddenly changes.

Sudden change in fluid velocity causes a pressure wave to propagate within the pipe. Under certain conditions, this pressure wave can create a banging noise, similar to that you would expect to hear when beating a pipe with a hammer. Hence the phrase.

Not surprisingly, common symptoms of this problem are high noise levels, vibration and broken pipes.

When a moving column of fluid hits a solid boundary – when a directional control valve closes suddenly for example, its velocity drops to zero and the fluid column deforms, within the rigid cross-sectional area of the pipe, to absorb the energy associated with its motion – similar to a car hitting a concrete wall.

However unlike a car, the fluid is almost incompressible so the deformation is small and a store of energy accumulates in the fluid – similar to compression of a spring.

The magnitude of the pressure rise that results from the subsequent release of this stored energy can be expressed mathematically as follows:

Pr = P + u p c

Where P is initial pressure, u and p are initial fluid velocity and density respectively and c is the speed of sound through the fluid.

Accumulators and other damping devices are sometimes installed in an effort to deal with this problem. However, the significance of the pressure rise equation shown above is that fluid velocity is the only variable that can be altered to address the root cause.

Put simply, reducing the velocity of the fluid column that hits the solid boundary, reduces the magnitude of the subsequent pressure rise.

Returning to the traffic crash analogy – the slower the car is traveling when it hits the wall, the less damage is caused.

In hydraulics, the easiest way to do this – on paper at least, is to increase the diameter of the pipe. This reduces fluid velocity for a given flow rate.

The other alternative is to control deceleration of the fluid column by choking valve switching time to the point where the pump’s pressure compensator and/or system relief valve reacts fast enough to reduce flow rate through the pipe and therefore velocity of the fluid.

Craig Cook