What is an 'O'-ring?
“A circular seal made from a material, that best suits the purpose to it’s intended use”
There are a number of materials that seals are manufactured from to my knowledge upwards of 20 basic types of material, but with specialist chemical additions this number is greatly increased.and This table is by no account complete …..but these are the types we come across most often when looking at U/W photographic equipment and are considered good for use with water.
Below is a table showing Seals used in U/W rigs
& what they are suitable for or not.
But does it matter what the seal is made from?
Most underwater housing manufacturers use a small range of materials for sealing their equipment, but the knowledgeable companies use a combination of materials to perform different jobs.
The humble 'O'-ring used in under water photographic equipment usually has only four job types that are required.
- Compression seal (squeezed to seal)
- Piston seal (slides & rotates)
- Friction 'O'-ring (drives or moves something)
- Locking Ring (holds fast)
Each of these functions utilises the 'O'-ring in a different fashion.
The Compression seal
The compression seal is actually a misnomer as most of the materials used for this job “cannot be compressed” but they can be “displaced” the ‘O’-ring material is moved under pressure to seal.
A compression seal can be found on any housing that closes onto a sealing surface. This is also commonly known as a static seal, some examples can be seen below
The Piston seal
A piston seal is set into a fixed bearing or onto a shaft that slides in and out or rotates. Often it will do both on a shaft or in a bearing, some examples can be seen below
The Locking Ring
Locking rings are a cheap and easy solution to hold items secure, they are normally forced into a space above an object that needs to be held securely and is occasionaly used on it’s own to stop water ingress, often another seal is used for this purpose. See below
The Friction Ring
A Friction 'O'-ring is used to drive a switch or wheel and relies on the natural grip of the material to physically force the operation of another unit, these 'O'-rings are normally relatively thick in section, see below for a common use
So what is the best ‘O’-ring for sealing my housing?
The manufacturer will normally specify a seal for a certain job, a soft seal to mould into a difficult shape or to displace to a greater degree, a harder seal to take more wear and tear.
Some manufacturers just copy somebody else's seal and appear not to understand why a particular material or hardness has been used, not surprisingly these brands tend to have the majority of problems associated with sealing or friction applications.
Some housing manufacturers use seals made from difficult to obtain materials or unusual shaped mouldings, this rarely improves the performance but will of course generally tie the purchaser into that brand for any replacements.
We have found over the years the very best manufacturers build equipment that use standard seal sizes and can be interchanged with locally purchased quality replacements.
This is not just down to the availability of spares but seals that are common standards have a better track record of use and are manufactured more frequently, so replacements are closer to the cure date. This gives replacement seals a longer service time.
Quality what quality? “A seal is just a seal”
Well not really, the material has to be of at least reasonable quality, it also has to be mixed to a good standard, the moulding tool has to be well engineered and the seals have to be baked to perfection.
Every time a seal is moulded a little ‘seeps’ out the edge of the mould and forms a ‘lip’ around the inside and outside edges of the ‘O’-ring, this is called the flash line.
The ‘flash line’ needs to be removed efficiently and the intended use will depend on the quality of the removal, for U/W rigs this must be efficient and clean, and the quality inspection should be thorough.
With this process a lot of issues can arise and to get really good aircraft quality seals can cost up to 100 times the cost of a cheap off the shelf seal.
Poor quality seals are generally made for general purpose engineering in a country with cheap labour and slack Q.A. although that is not always the case.
How can I tell if my seal is a quality seal or cheap rubbish?
This is a great question and is so often overlooked by users and some manufacturers alike.
The flash line is the first place to start, use a little ‘O’-ring lubricant on the crook of your finger and run the seal around lightly gripping between your thumb and forefinger.
The flash line will feel smooth and you will barely notice it, or it may have a small but defined ‘lump’ part way round, or it may feel rough & lumpy all the way round in this case recycle it and find another supplier.
If it is has a small lump or smooth it’s worth a visual inspection.
A small lump is often just a flake of material that has attached itself during the processing, but will often just rub off with a little pressure. If it doesn’t rub off it can often be removed with a fine emery board, but never cut away a proud flash line, after removing any lump it must feel smooth.
Smooth should not just ‘feel’ smooth but also ‘look’ smooth, no dips or hollow marks around the moulding joint.
The other problem we come across with new seals is where they have been moulded from a single strip of material that is laid into the mould, to be joined together during the baking process.
If this is not done accurately enough, it will form a slight difference in material thickness at the join.
This may feel like a lump, but if the seal is gently pulled it shows as a dip or sag in the material.
If this is the case recycle and find another supply.
How can I tell if my seals are past their best before date?
Manufacturers of seals always produce quantities of seals at a time and when they do a batch number is created.
This allows buyers to source information on the cure date (date of production). Each type of material has a different life span before the seal begins to show signs of age, so manufacturers will give a best before date for each seal batch.
If the seal is new and in storage it should be kept at constant temperature, a set humidity and not be subjected to harmful light (usually UV) or harmful gases.
The same is true if you have purchased spare seals unless you treat them in the same way they will degrade much faster than the best before date.
As an example the most common seal NBR will have a life span of around 7-14years depending on who you talk to, for silicone seals it’s up to 20 years.
When the NBR seal was first produced, the life span was 6 years, this was listed as 2 years storage, 2 years optimal use and 2 years ‘wriggle room’ or waiting time for service and replacement.
With the improvement of storage facilities the 6 years has been extended. But not all manufacturers have full control of the environmental conditions for optimal storage.
Which is why we always work with a seven year window and we always ask for seals cured no longer than two years previously.
This will give a storage service life of up to five years, most manufacturers suggest a service every year, and most of our customers get equipment serviced every two to three years. We find it’s generally not a great idea to leave it three years as the environment we use our kit in tends to pay a heavy toll on all moving parts of the rig it can get into.
So how do I know what grease to use on my housing seals?
Basic differences in grease are very hard to see, obviously some are much ‘thicker’ or stickier than others, they come in a range of colours & hues.
But it’s the chemical make up, that stops them being absorbed into the materials they are designed contact with, or from being flushed out of the system when immersed in the type of liquid they are designed to be used in.
For the our purposes we can assume they are all used in fresh or salt water, but in industrial situations this may be different.
As a general rule of thumb any grease that has the same ‘base’ as the material it sits next to, will be absorbed into that material.
The most common for U/W photographers is silicone grease being used with silicone ‘O’-rings, this will cause the ‘O’-ring to swell as it absorbs the silicone oil & it will eventually go soft & saggy, I think most of us have seen this play out at some time.
The same happens with petroleum greases used on natural rubbers, it will degrade them rapidly.
But seals made from Nitrile or Chloroprene (Neoprene) will operate without problems in petroleums.
Knowing the base material of the grease is generally not information we as photographers are party to.
So it is always best to use the manufacturers own brand of grease…..
The only exception is if you are using an O2 grease, these greases are totally inert so will have no chemical interaction between your housing materials or the seals you are using.
The problem comes when you need to clean your seals!
This can be a big problem, especially when washing kit down in say, warm soapy water.
As many of the soaps we might use will act as a ‘degreaser’ great for dishes, but not for greased ‘O’-rings.
This is fine if you have taken the ‘O’-ring out & you are cleaning it.
But the ‘O’-rings in the control shafts should not be subjected to the same process, as it’s not an easy process to re-grease these seals efficiently.
So it’s best to just use warm water when washing down the whole rig.
But how should I clean my ‘O’-Rings?
If you can remove the ‘O’-ring you can actively clean it, degrease it and re-grease it, knowing what the seal material make up is, will help you avoid the chemicals that will destroy your ‘O’-ring.
In our underwater photography world we have only a few basic types of seal materials.
- Nitrile Butane rubber
We would recommend each time using warm water and a dry cloth, this is often all you need to remove any detritus stuck to your ‘O’-ring seal and the existing grease.
For heavily soiled seals, all of these materials will tolerate cleaning in mild warm soapy water using general household washing up liquid.
It’s can also a good idea to clean your seals with the base oil used in the grease, this can sometimes be skimmed off the grease from the top of the tube.
If your seals are badly stained or have deposits that are hard to remove using the methods above, then using something more harsh as a cleaner has to be done with thought and quickly, any seal will suffer if exposed to harsh chemicals for an extended period.
Most seals used in underwater camera kit will tolerate cleaning with a refined fuel like lighter fluid, but we would recommend only applying via a cloth as any residue will evaporate rapidly.
Cleaning the seals you cannot remove is best done using mildly warm water only, any de-greasing will require the seal to be re-greased in situ.
This is only possible if by applying grease to the inner shaft, you can transfer it to the ‘dry side’ of the seal, so the shaft/control has to move in/out and rotate.
Applying grease to the outside of the seal, acts better as a detritus magnet rather than a seal lubricant.
Many divers use products like oil sprays to keep things moving in their U/W rig, metal to metal is generally OK.
But don’t deliberately put it onto any of your rubber components, or get it near the plastic windows, without knowing what chemicals are in the spray product and what the propellant that expels it are made from, these must be compatible with the ‘rubber/plastic’ you are applying it to.
Also bear in mind, that any light covering of oil will generally be removed rapidly with flushing water during your dive, so a suitable grease may be a better option as it will last a lot longer.
I hope this brief explanation has helped you understand a little more about grease, the relationship between the components that are used in manufacture and a glimpse into the world of Tribology.
A fascinating subject that supports almost every single thing that moves, that we use today.