The squeeze and shrinkage on an o ring are vital to understanding as these factors stand in the way of your application either experiencing a perfect seal or your application suffering from leaks.
You can measure the shrinkage of an o ring during the moulding process, where we look at the percentage difference/range between the mould material and the thermal expansion.
This percentage is what is known as the shrink rate.
O ring suppliers throughout the UK and specialistsealingproducts.co.uk work with precision engineers and gasket manufacturers to provide a great deal of information and advice surrounding o ring squeeze to help ensure perfect application design.
If we take a look at the o ring calculation, we can see that:
Shrink rate = dimension of the cavity – the length of the part / the dimension of the cavity dimension (ie the typical units)
Providing you with a shrinkage value that is essential when you’re looking to scale up the size of the finished part to the cavity, i.e.:
Cavity dimension, therefore, equals: the finished part length / 1 – rate of shrink
- In hydraulic operations, a squeeze percentage of between 13-27% is typical.
- In pneumatic operations, a squeeze percentage between 10-24% is the norm.
(Note: These percentages will vary depending on the materials in use, temperature, and other environmental influences).
Material shrinkage, however, isn’t easy to predict, and there are many variables to consider that will affect how you shrink rubber o rings.
For example, the moulding method can influence o ring squeeze due to compression, transfer, and injection. These elements alone are known to affect viscosity flow, polymer orientation, and the overall shrink when the rubber is vulcanised.
The good news is adjustments can be made during the processing cycle—adjustments such as temperature, time, pressure, and speed.
How to shrink rubber o rings
When we examine our processes in more detail, we see that higher mould temperatures can lead to higher shrink rates due to the thermal expansion the mould material experiences.
Additives such as fillers, curatives, and lubrication can also affect shrinkage depending on your requirements and the application in question.
This is why testing is always recommended. Testing as close to the actual production environment as possible, helping to validate the moulding process and check that finished part sizes are correct for the intended application. At the testing stage, you can make appropriate adjustments, ensuring that o rings fall within the specified tolerance levels.
The importance of o ring squeeze
O ring squeeze is vital in creating successful seals for applications, where we define squeeze as the amount of pressure o rings are subjected to when placed between the hardware and then the extent of deformation that will potentially occur.
Designed to provide the perfect seal, these gaskets are used to occupy the space between mating surfaces, preventing any leaks from occurring. O rings in this sense must therefore be bigger (cross-sectional) compared to the gland in which it is placed.
When o rings are compressed they will expand in the gap and form a tight seal within the application.
But how much squeeze should be applied?
This all depends on your application and the size of the sealing gasket.
For example, dynamic applications require a lesser squeeze because this helps to reduce wear on the seal due to an increase in friction.
However, in applications where gases must be contained, a higher squeeze should be applied to prevent leaks, as increasing the squeeze percentage will tighten the seal.
(To understand and find out more about the squeezing force behind o rings, make sure to check out our post on how much squeeze is enough, here.)
Ultimately, a higher squeeze will result in a tighter seal, but be cautious because more is not always better.
Why?
Because the more force applied during installation, the greater risk there is of developing a pinch creating an unfortunate opportunity for leaks to occur as the o ring and hardware are placed under excessive stresses.
Higher squeezes also increase the risk of additional friction, creating faster wear on the application.
That’s why the team at Specialist Sealing Products recommends that squeeze is optimised, not maximised.
To find out more and for a range of o rings and gaskets available, call us on 01535 274 776.
