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Ever tried to decipher a hydraulic circuit diagram and make sense of its symbols? Our blog will help you to understand the meanings and functions of the common ISO1219 hydraulic symbols.
As there are so many possible combinations of system parts and functions possible, we look at the base component circuits that make up a typical hydraulic power unit.
A hydraulic pump typically comes in either a fixed or variable form with the variable version allowing adjustments in flow rate and outlet pressure. They both have the same aim though and that is to pump oil from the hydraulic reservoir back into the system.
• Hand pump – These pumps are handheld mechanical devices which pump high pressure fluid in one direction
Filters are an essential part of any system as they help filter our particulate from fluids and, in turn, they keep components in good shape and the system running effectively. They come in different sizes, with some in cases and functions and can be placed almost anywhere on a hydraulic unit if they are of the pressure filter type. Another popular type of filter is the return filter when filter oil is deposited back into the reservoir.
All in all, filters help to maximize the service life of a system provided they are changed at the specified manufacturer recommended intervals.
The role of a pressure relief valve is to transfer fluids from areas of high pressures to those of low pressures. This most likely involves the tank.
In the first part of this blog we covered a range of common hydraulic symbols, explaining how they formed part of a circuit diagram along with their various functions. We continue where we left off, focusing on common hydraulic valves.
The pressure control valve comes in two basic forms; direct acting and pilot operated and the main function of these is to control the flow rate or its pressure. As there are a number of different types of pressure control valve which deal with variations in pressure their symbols can appear very similar. A good way to check which variant is used in a system is by the location of the valve in a hydraulic circuit.
The directional control valve is responsible for controlling hydraulic fluid flow. The spool of the valve works with the valve body which opens and closes the internals to control fluid flow.
- Actuators are always responsible for the push and never the pull of spool
The hydraulic check valve works to prevent flow in a certain direction. A spring in the check valve enables the valve to open but only when the pressure is exceeded. Reverse flow can be attained by the valve opening under the influence of pilot pressure. This is usually the case if one was looking to hold the pressure in the hydraulic cylinder.
In order for a hydraulic system to store its fluid a reservoir must be employed and these come in various forms including closed and vented tank forms.
Vented tanks are more commonly used in general applications with the closed variety mainly used on offshore and aviation industry applications.
A hydraulic cylinder operates by generating mechanical force through hydraulic power. The cylinder illustrated above is a typical double acting welded end variety and, having two ports, can be powered in and out.
How to read hydraulic circuits
Hydraulics symbols are an essential component of hydraulic circuit diagrams. Knowing some of the basic principles will help understand a wider range of symbols. Explaining the common ISO1219 symbols enables a complete hydraulic system to be followed:
1. Hydraulic Pump
Hydraulic pump produces flow. Oil is pumped from the hydraulic reservoir into the system. The basic symbol for a pump:
A fixed displacement pump is the simplest type and has a fixed output for each revolution of the input shaft. Modifications to this symbol describe the variable displacement pump. The types of control circuits show how the output is varied.
Filters clean oil entering the system, and are used in various places within a system. They protect hydraulic valves and pumps. Suction filters are placed at pump inlets to ensure only clean oil enters the system. Pressure filters can be placed throughout system. Return filters are common and filter oil returning to the reservoir.
3. Pressure Relief Valve
Pressure in a hydraulic system should be limited to control the force any motive devices produce and to ensure the safe/design limits are not exceeded. A pressure relief valve symbol is normally shown as:
A pressure relief valve or PRV passes fluid from an area of higher pressure to a lower pressure (typically the tank). Hydraulic pressure shown by the dotted line acts as a pilot to actuate the PRV by moving the arrow across the box. This happens when the pilot pressure produces an internal force equal to the spring load the valve begins to open and pass flow.
4. Check Valve
This valve is a one way valve that prevents flow in one direction. The addition of a spring ensures the valve will only open when this pressure is exceeded. Dotted pilot lines can be added so that pilot operating pressures can be used to open the valve and allow flow in the reverse direction. Commonly used to hold pressure in a hydraulic cylinder.
5. Hydraulic Reservoir (tank)
Hydraulic systems all have a means of storing hydraulic fluid. This is referred to as the hydraulic reservoir. Hydraulic reservoirs are shown as:
Vented hydraulic reservoirs are common place, but sealed systems can be found ion aerospace and marine applications. The return lines shown indicate the position above or below the oil level.
6. Directional Control Valve
Hydraulic fluid flow is controlled by a directional control valve. Commonly consists of four parts, valve body, spool, actuator, and springs. The spool is moved with respect to the valve body, this opens and closes internal flow galleries to control fluid flow. Various types of actuators provide power to shift the spool and springs are normally used to return the spool when the actuator is de-energised.
Look at the typical three position four way valve:
How to read directional control valve symbols:
a. Each box in the valve symbol represents a possible valve condition. In the three position valve above there are 3 possible conditions controlled by the actuators.
b. Number of ways tells you how many hydraulic connections could be connected to the valve.
c. Actuators always push and never pull the spool.
d. The box furthest away from the actuator is the normal or de-energized position, and is the position where the circuit connections are drawn. In the above valve this is the middle position.
7. Hydraulic Cylinder
Hydraulic cylinder or actuator uses hydraulic power to generate mechanical force. A hydraulic cylinder is shown as:
A double acting cylinder (above) has two ports and is therefore powered in and out. Single acting cylinders have one port and would typically be used for lifting applications.
We hope this gives you a useful introduction to hydraulic circuits. For a full list of hydraulic symbols can be found in ISO1219, or contact www.hydraproducts.co.uk for more help.
There’s no doubt that having an accurate diagram of your hydraulic system setup can save many an engineer considerable time and trouble. Here are the main diagrams that you might want drawn up of your system to refer to when needed:
Block Diagrams using blocks joined by lines, this diagram illustrates how the components are installed in addition to their connections and interactions.
Cutaway Diagrams illustrate what’s inside of hydraulic components including flow paths. Colours, patterns and shades are used to represent flow paths and pressure.
Pictorial Diagrams present the arrangement of the components and pipes of the hydraulic circuit. Usually this diagram shows the shapes and sizes scaled to help with component identification and recognition.
Graphical Diagrams simple symbols are used to represent each hydraulic component, their connections and controls.
Hydraulic technicians who have experience at interpreting circuit diagrams find these a great resource for understanding what could be at the core of any problem for troubleshooting purposes.
Without the schematic diagram, the hydraulic engineer must work hard to comprehend the design of the circuit and use isolation to identify potential origins of issues. At best, this can take a lot of time and energy that needn’t be spent.
In worst case scenarios, a valve manifold may need to be dismantled and removed in order to understand what its purpose is. Without a full understanding of any component in a hydraulic system it’s difficult to know exactly what is causing an issue. The purpose of the schematic diagram is to remove the necessity to reverse engineer the hydraulic circuit.
Do you use hydraulic circuit diagrams to support maintenance and repair in your business? If so, which types do you use to enable your engineers to gain a fast and complete understanding of your systems?
Hydraulic engineers and designers have long been facing uncertainty over the future of hydraulic power in an increasingly electrified world. The replacement of hydraulics with electrical actuators and components in vehicles especially has caused unease over whether hydraulics truly has a place in effecting motion as technology marches forward. With the introduction of the Internet of Things (IoT), a truly connected network of equipment, household appliances and even lighting or heating systems, hydraulic power seems outdated, and likely to be replaced by electric alternatives as the technology improves unless the new generation of designers embrace the benefits of hydraulic power.
Hydraulics still has many clear advantages over electric power; load bearing capabilities and predictive maintenance are just two of the benefits of using hydraulic power and easier troubleshooting and repair underline the plus points of hydraulic versus electrical power. It is obvious, however, that for hydraulic power to survive and compete it must integrate seamlessly into electrical circuits so that there is no reason not to choose a hydraulic component over an electrical one, solely on the basis of ease of integration into the rest of the system. Many hydraulic power packs now, including the ones produced by Hydraproducts, are designed to fit with electrical circuits and to be used with electrical power, translating a small amount of electrical power into a much larger hydraulic force, without any risks of high voltage electrocution or shorting out a circuit under increasing loads.
One of the most understandable facets of the IoT is the unmanned warehouse. Already in trial by Amazon (using drones) and some Chinese companies (using robots running on a grid matrix), these automated warehouses need minimal human staff, with even deliveries being accepted by robots using RFID tags. The central processing office can oversee the delivery, but no one needs to physically sign for a consignment as this can all be done through sensors. Moving new stock to the right location within the warehouse is done through robotics, and the incorporation of electrohydraulic components means even heavy items can be moved and lifted into place on shelves. Sensors ensure that the location of each item is logged, and this data can be used to create an automated picking list for the same electrohydraulic robots to compile an order. Electric actuators may be used for warehouses that only deal in lightweight stock, but for car parts warehouses and other stockists of heavy components the extra power that hydraulic components offer is essential for true automation.
If nothing else makes hydraulic components an attractive choice, then the ability to scale up power and force through the intelligent use of hydraulic power certainly does. Electric alternatives may be getting cheaper and are undoubtedly easier to wire into a circuit than traditional hydraulic units, but the marriage between electric and hydraulic power makes perfect sense for fully capable robots that can cope with lifting and transporting items of all sizes and weights. The replacement and maintenance involved with fully electric systems is comparable to that of an electrohydraulic system, but it can be much harder to pinpoint the exact cause of a problem without careful study of the wiring schematic and an understanding of the original design. Hydraulic components, by comparison, are easier to fix for those who were not involved in the design process and given that engineers and maintenance people generally are not involved in the specification of a system, it is intelligent to have a system that can be fixed more easily.
The IoT is not confined to commercial and industrial applications, however, and in part 2 we look at the uses in the smart home.
Hydraulic filtration is a vital component of keeping a system running smoothly.
For example, did you know that up to 75% of failures with fluid power can be attributed to contamination? With the use of hydraulic filters, contamination damage can be significantly lowered which can not only cut down on expense but lower that 75% drastically.
If you’re looking to save costs from less downtime then it’s also time you looked into what a difference hydraulics filtration can make for extending the life of your equipment. Running your system optimally is essential when it comes to cost saving, but protecting its longevity is also a critical element in running any business efficiently.
Muck and dust can destroy a hydraulic system, that’s why it’s essential to make the best use of hydraulic filters. You wouldn’t even be able to remove that dirt yourself, as it’s likely to be dust that is so fine that you won’t be able to see it without the use of a microscope. Dirt has the same detrimental effect as sandpaper or gravel and not only will generally deteriorate the system, but it could even destroy it.
However, through the use of a hydraulic filter system you will be able to maintain control over the level of contamination and by doing so reduce the failure of systems by as much as 75% just be removing that dirt.
Hydraulic parts are expensive. Combine that with down time and having to keep engineers on hand to fix worn components and that’s a lot of expense to deal with. Putting filters into place can even save costs by increasing how long the hydraulic fluid will last.
Degradation of fluid – hydraulic fluid that contains fine metallic particles can degrade rapidly through chemical breakdown. Without protecting against this, there could be issues such as slippage, internal leakage, corrosion or sticking parts.
Scoring of surfaces – this can occur when particles get trapped between surfaces of seals
There’s no doubt about it, but …
· System performance is affected by dirt levels
· Hydraulic filters can control levels of dirt. Without using this management method, the system will get dirtier and dirtier until it fails.
In fact, hydraulic filters are the only way to control how much dirt is in fluid. Without them you will be forced to change out the hydraulic fluid regularly, which can be a time consuming and costly event.
Hydraulic system dirt particles are incredibly small. In fact, they are so small that they cannot be seen by the human eye – and 98% of hydraulic fluid has some dirt in it.
Engineers have found that when it comes to size of particles in samples taken from operating systems, the smaller the particles, the more dirt there is in the system.
So where do these particles come from that we have to work so hard to deal with?
In order to have an idea of what goes on inside the closed system, let’s examine where these particles come from.
Instead of enjoying the typical 20 gpm that is the measurement of a pumped flow from a 2000 psi system, you can expect to see something in the region of just 10 gpm. Although your pump will still produce for you, you’ll discover that the degradation results in just 50% efficiency and you should als be prepared to experience extra heat and other unwanted issues.
As with any hydraulic system, there is an optimum level of cleanliness, but there is a point where you cannot get any better performance out of the system by improving the quality of the fluid. However, with the use of hydraulic filters you should be well set to extend the life of your machinery.
Working with hydraulic machinery involves a certain degree of trouble shooting and problem solving, even people who operate the equipment and are not involved in maintenance and repair, should have some basic knowledge of what can go wrong and what to look out for. When it comes to diagnosing and solving problems with hydraulic machinery there are several things that should be checked, and just as many tips and tools to help you do so.
Firstly, if a schematic diagram of the equipment is available this is a very handy tool for fixing problems. It may be obvious that a seal has gone somewhere along the line, but without a schematic diagram it can be a long trial and error process of finding, then changing every seal in a process of elimination. On very complicated and large hydraulic equipment with multiple hoses and cylinders it helps to be able to quickly locate each junction, especially if the presentation of the issue points to where the problem may be located. The schematic diagram also allows you to identify the potential causes of a problem, and prioritise them by likelihood, meaning less time is spent speculatively replacing parts at random and more time is spent actually investigating the most likely causes and addressing them first.
A flexible powerful light is also very useful for finding faults in hydraulic equipment, as some labels and parts may be out of the way and hard to see or read. Big torches are powerful but not so good for tight spaces, so a fibre optic light is a good investment. Magnetic dropper tools are also very handy when disassembling complex parts in situ, as if a small part falls into a reservoir or cylinder it means taking the whole thing apart to retrieve it, and in the worst cases replacing that part entirely, which can be very costly. A small magnet on a telescopic rod may be cheap and small, but can come in very handy indeed.
Once the potential problem has been diagnosed, the offending component needs to be removed and inspected for faults. Compressed air and air guns are great for cleaning off parts and for inspecting the integrity of valves. Similarly, automotive brake and clutch cleaner is an invaluable tool for cleaning greasy and contaminated components, without the need for pressure or rubbing that could otherwise damage the part. If the suspected part turns out not to be the culprit then it needs to be clean and free from contaminants when it is replaced, otherwise further issues could arise from dirty parts being replaced in the machinery.
When disassembling parts it is imperative that all components are inspected and kept visible; a lost screw or other vital part can cause bigger problems than the initially broken part if it is hard to replace. Using heavy white paper to lay out the components keeps them in sight, in order and makes it easy to see oil leaks, as well as keeping the oil off other things in the vicinity. It is also handy to be able to make notes on the paper, circling components that are okay and crossing next to ones that are broken.
Having a small tool set to hand is vital for repairing faults, and it is worth getting any specialist tools that the hydraulic machinery may require so they are to hand when needed. Spare seals are also vital to have on board, as these are often the cause of leaks but should be replaced when inspecting and repairing faults, even if they are not to blame. It is a good practice to replace these if they are removed for fault finding, as they could be compromised and stretched from being removed.
Join us in part 2 for more hydraulic trouble shooting tips that are firmly in the 21st century.
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