Hytork® springs are designed to the Spring Research And Manufacturers Association (SRAMA) criteria for reliability and have sufficient safety margins for their use as a safety device. The designs are backed up by a tight manufacturing Quality Control System, certified to ISO 9001 which is supplemented by a continuous random testing programme.
- Spring Return Actuators are "Fail Safe"
- "Fail Safe" devices are used only for safety concerns:
- Any design with a predictable Spring life less than the plant life is not an acceptable "Fail Safe" design
- Hytork® Springs are designed and guaranteed never to break!
Sectional Diagram
The load is distributed evenly around the whole circumference of the nd Cap, preventing distortion and point loading stresses.
This SafeKey system also prevents the possibility of overstressing bolt hole threads during assembly. Click here to see how the SafeKey system works.
The Fail Safe function of springs is the prime design specification and Hytork® springs are designed accordingly.
Hytork® engineers believe that aesthetic compactness of design with associated increases in spring stresses is secondary to the prime requirement of continual safety: this is why Hytork® springs are large, strong and reliable.
Corrosion protection: Hytork® springs are also epoxy coated to protect against corrosion.
Hytork® springs are epoxy coated to protect against corrosion.
Hytork® does not offer springs as a spare part, as they are guaranteed for the working life of the actuator with no cycle limitations. To offer spare springs would be a contradiction in our design criteria for maximum safety.
We are so confident in our product that we will replace or repair any actuator that has a spring failure in normal service, free of any charge, irrespective of the date of manufacture.
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Hytork® springs can be selected to match a wide range of air pressures to optimise actuator efficiency. All Hytork® springs are designed to the same high standards.
A Spring Return Actuator uses air pressure to move the internal pistons and also compress the spring to store energy for the return stroke. As the springs compress there is an increase in the power consumed by the springs and a consequent decrease in the torque available for operation of the valve or damper.
The ideal solution to overcome this would be to make the power from the Springs equal to the power from the air in all positions throughout the actuator's stroke (the 50/50 relationship).
For a spring to produce the same power at the end of its working stroke (WS), as it does at the beginning, the spring must be so long that the working stoke is insignificant when compared with the total free length of the spring. (See Fig. 2).
In other words, the spring would need to be of infinite length. This is impracticable, so a compromise must be made.
For example, two currently marketed solutions have been:
a) Go for a very long spring In order to make the drop off in power from the spring as small as possible.
b) Go for a very short spring which will give a very rapid drop off in spring power output, but a very short actuator.
Long Springs: The problem with a very long spring is that although it is closer to satisfying the 50/50 ideal, it produces a heavy, cumbersome actuator. Some designs using this approach are asymmetric, having springs at one end only. This creates problems for handling and installation and often interferes with other equipment or passage ways.
Equally important, the offset loading on the mounting can lead to pipes twisting, premature valve stem leakage and a requirement for support brackets.
(See Fig. 3).
Short Springs: A very short spring appears to solve the weight and space problem. However, as very short springs mean that the output power decreases rapidly, it is sometimes necessary to increase the size of the actuator to compensate for the power drop off. Obviously, if it becomes necessary to increase the size of the actuator, the apparent benefit of a short actuator is completely lost.
High stress levels in short Springs: in order to gain the maximum power from the short Springs, some designs use stress levels above those recommended. The effect of this is that the Springs can be overstressed and will predictably fail in service.
Fail Safe Actuators are significantly more expensive than double acting models. However the more closely an actuator's performance can match the valves requirement, the more cost effective the actuator will be.
Hytork® spring designs are engineered to make the solution match the application.
Hytork® is also fully aware that the end users need flexibility. The Hytork® design permits end users to alter the spring characteristics safely and easily as required, without having to return the actuator to the manufacturer.
Diverse types of quarter turn valves have significantly different operating torque characteristics. Plants have requirements for both fail open and fail close valves. The air supply pressure available at the actuator location also effects the actuator size.
Hytork® springs are designed to give the user flexibility for whatever application and operation is required. The actual selection of the correct spring combination can be achieved by either referring to the torque charts given in the literature on spring return actuators.
Information for spring adjustments for different air pressures is available on this data sheet.
Correct actuator sizing is critical in ensuring safe and effective plant operation. As an aid to our customers, Hytork® recommends the use of the Hytork® actuator sizing manual. The manual not only makes selection more accurate, it is easy and quick to use.
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Hytork® has made changing springs on site both simple and safe by the combination of the End Cap SafeKey Retainer System and the Spring Removal System. These very important features protect and facilitate the technician when springs have to be changed to adjust for variations in air supply pressures to the actuator.
The force of the springs locks the SafeKey in place and does not allow the end cap to be removed until the spring is safely captured by the spring retractor rod.
The retractor rod is inserted through the end cap and screwed into the retractor cap.
The nut on the rod is then screwed against the spring end cap to draw the spring away from the piston. This action contains the spring force within the end cap and releases the locking force on the SafeKey.
Once the locking force has been released, the SafeKey is free to be removed and the spring return end ap assembly can be safely removed as a module, from the actuator body.
For normal maintenance, when spring changes are not required, it is not necessary to completely relax the springs. The spring end cap module can be left with the retracted rod holding it together ready for re-assembly.
If it is required to alter the spring rating, the nut on the retractor rod is gradually unscrewed until the spring is in its completely relaxed position. Once the spring is fully relaxed the retractor rod can be removed and the springs changed as required.
- Springs are "Fail Safe" by design and guaranteed for life.
- Springs are epoxy coated for corrosion protection.
- Springs are easy and safe to change on site.
- Springs can be easily adjusted to match valve torque and site air pressures.
- Actuators are balanced around the center of the valve stem preventing pipe twisting.
- Springs are produced to suit high and low temperature environments.
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