One of the most important aspects in ensuring proper performance and longevity within your hydraulic cylinder is having the correct size seals for the housings that you have. Too frequently the attitude of “its close enough” results in a poor outcome with issues such as bypass, leakage and misalignment occurring within the cylinder bringing with it another costly repair and loss of operational hours. In this article we cover how to measure your hardware, tolerances and choosing the right seal for your cylinder.
There are two ways that seals can be measured and only one is the correct way of doing so. Measuring from the old seals removed from the cylinder poses risk. These seals are worn and damaged, and may not have been correct for the application the cylinder is used in the first place. Hydraulic Seals Australia strongly recommends only measuring from the steel hardware and only after such time where rust, pitting and other issues are removed and housing sizes are final.
MEASURING EQUIPMENT NEEDED (top to bottom):
External Vernier
Internal Vernier – preferred over traditional calipers
Groove width caliper
First point in identification is to determine if the cylinder is Metric or Imperial. This can often be clear if the origin of the equipment is known, i.e. Hitachi being from Japan will always be metric, however this is not fool proof as brands such as CAT® use both metric and imperial sized rod and bores in their cylinders. The easiest step in identification is to measure the rod (shaft) and the bore (barrel), determining this at the start of your measuring will allow for a easier process moving forward.
Once the rod and bore sizes are confirmed, its time to begin measuring!
Starting at the top with the gland, we need to identify if a flexible wiper or a press fit wiper is required, a flexible wiper has a shoulder that holds it in place (this shoulder is as the retaining diameter), while a press-fit wiper fits into an open housing. we can see with the gland we are measuring that it requires a flexible wiper. Using the internal Vernier, we measure the D1 diameter (OD) of the groove, ensure the vernier are level and not slanted and then record your measurement. Using the groove width calipers we measure the L1 (width) of the groove, again ensuring that the calipers are level and that the points are sitting flat with the two end surfaces, this will ensure accurate measurements are taken. We then measure the d2 diameter (retaining).
Measuring D1 (OD) Measuring L1 (Width) Measuring d2 (retaining)
Moving internally down the gland, the next groove to measure is that that holds the rod seal. This requires measuring and recording of the sizes for the D1 (OD) diameter and the L1 (width) as has been done for the wiper seal. It is important to note that the landing behind the seal is crucial to selecting the correct seal and must also be measured. This measurement and the resulting clearance will allow us to calculate the extrusion gap and determine the seal required for the longevity and smooth operation of your cylinder.
Measuring D1 (OD) Measuring L1 (Width) Measuring landing behind seal
The last required measurements for the gland we have in front of us is to use the external Vernier to measure and record the d1 (ID) of the O-Ring groove and the L1 (width) of the groove. As this groove is external, we can use the external vernier to measure the L1 rather than the groove width caliper as we have used for the internal widths. It is worth noting that some glands will “step up” as they go into the bore so it is a must that the landing where the O-Ring seals against is measured.
Measuring d1 (ID) Measuring L1 (Width) Measuring static seal landing
It is important to note that each cylinder has its own layout based on the operational requirements set out by the original manufacture. This cylinder we are measuring today is a simple AG cylinder, comparing this to a excavator cylinder, there is no buffer seal and no DU bush. The piston arrangement we are about to measure also is substantially different and while the layout and contents of the cylinder may change, the same principals apply for the measuring of each groove.
With the gland measured, we now move onto the piston. This piston in front of us has two grooves, one for the wear band and the other for the piston seal. Using the external calipers we need to measuring and record the d1 (ID) of both these grooves, as well as the L1 (width) of them. Pay close attention when measuring the d1 (ID) as in some instances, the jaws will sit up on the radius in the bottom of the housing groove, resulting in a incorrect measurement. As with the rod seal, we also need to measure the landings on either side of the piston seal groove. This will determine our clearance and subsequent extrusion gap and allow for the correct selection of piston seal for our hardware sizes.
Measuring d1 (ID) Measuring L1 (Width) Measuring landing behind piston seal
Measuring piston wear band d1 (ID) Measuring piston wear band L1 (width)
Using Hydraulic Seals Australia’s website, we can begin to select and identify the seals that we require for the hardware sizes that we have measured. With each item that we have measured, there is an allowable tolerance on the d1 (ID), D1 (OD) and L1 (width) above the nominal size stated and with pressure seals, such as the piston seal and rod seal, there is an allowable tolerance for the extrusion gap based on the operating pressure of your cylinder. These tolerances can be found on the product technical data sheet available for each seal profile at https://hsa.com.au/pages/technical-downloads
Website selection process.
To make the website selection process clearer, here’s a simple step-by-step breakdown:
- Go to the Hydraulic seal category on the website.
- Filter by imperial or metric, depending on your measurements.
- Filter by the size you’ve measured (rod size, bore size, etc.).
- Browse through the results and pick the closest match.
- Click on the item to open its product details.
- Check the product details and the technical data sheet to confirm the seal fits your cylinder’s specs.
Following the above steps, we can identify the wiper seal closest to our needs as being the AN-01500 wiper.
While our measured housing L1 (width) is larger than the nominal specified as above (0.107) at 0.109, referring to the technical data sheet confirms that this is within the allowable tolerance and that this wiper will fit our hardware (view technical data sheet).
This same process is used to identify the rod seal noting the necessary extrusion gap for the operational pressure of the cylinder being a maximum of3600PSI. Having measured the landing behind the seal at 1.507, and knowing our rod size is 1.500, we have a extrusion gap of 0.007”. This gap is diametrical and not radial as stated on the technical data sheet. To get the radial extrusion size, we divide the diametrical extrusion size by 2, 0.007”/2=0.0035”. Using the table on the technical data sheet, we locate our seal cross section and max operating pressure of the cylinder. Where these two lines meet will tell us the max permissible extrusion gap for the seal we selected. In this case, this seal states a max radial extrusion gap of 0.008”, larger than the 0.0035” we have allowing for the use of the selected seal in this cylinder.
The same steps are then followed for the remining required items in our cylinder.
Using this process allows for the accurate selection of seals based on housing sizes measured and ensures that the seals selected will meet the pressure requirements of the cylinder. Using a seal not fit for purpose will undoubtedly result in issues with your cylinder. For seals that do not meet the allowed tolerances, HSA can offer same day on-site custom machining using one of our many CNC lathes we have located in each of our branches made specific to your hardware size.
