Cutting through to hydraulic cleanliness.

Hydraulic Cleanliness is essential.

Design the contamination out. Some designs and materials are just a battle to clean.  What have I always struggled with? 

Nasty little grooves harbour even nastier particles of contamination. Contamination can be impossible to chase out of a groove that you cannot look into. Give serious thought to using open grooves on designs where a stepped bore can be machined like a dream, inspected, a seal fitted then a closure popped in to form the seal groove. If you are lucky, this can be mechanically retained rather than a threaded part. Yes, cost is an issue but you cannot put a price on unreliability.

Last chance strainers are useful on inlets and outlets if only to decide that a lump of something came from the customer’s kit, not yours. It can be tempting to choose a very fine strainer, but we suggest using at least 250 micron stainless wire mesh.

As for cleaning out horribly complicated manifold blocks, Sarum thinks that thermal deburring was one of the best processes that we ever found with the proviso that you still have to deburr the part properly. The residue powder has to be removed, though, unless you are anodising or plating.

Reservoirs that have too small holes and that you can never clean out also need special attention. Reservoirs with removable lids or a manhole plate are quite onerous and bring different problems if rigidity is not there. If you can get a decent size of hole into a reservoir, then you can achieve cleanliness. Blowing muck around in a reservoir isn’t the best of ideas. Flushing is OK provided you have a really turbulent flow that will slosh around every scrap of fluid and scour out muck. Hoovering a reservoir out isn’t a bad call in the first instance.

What if your plating tanks are not clean?  Some plating tanks are just grim and your beautifully plated kit can become truly filthy. Maybe water or solvent flushing will clean them up at your works.

Help to help yourself before you have even started assembly.

• Don’t run a machine shop and a hydraulic assembly shop in the same area. The regime of white bench tops with no junk on them and a light epoxy painted floor should give an assembly area clean enough to eat your dinner from.
• Washing work tops / brushing and mopping floors helps control muck from swirling around.  Beware of hoovering and even worse, air-blow guns which simply blow filth everywhere. They are an unmitigated disaster.
• No cardboard, no food, no air guns blasting muck around.
• Cover pots of grease when not used.
• Cover work up at every opportunity to stop airborne muck from entering.
• Use cleanroom wipes, not paper towels.

Contamination problems are more likely to be introduced during assembly than from dirty oil. What’s more, flushing may well not fix it. You may just be asking too much. Here are Sarum’s top pointers for hydraulic cleanliness:

You need clean parts and for this, an ultrasonic cleaner is an incredibly handy tool. For small parts Sarum is sure they are a good call. In addition, clean solvent and cleanroom wipes plus an eyeglass to look carefully at components are Sarum’s hot tip.

Don’t place too great a reliance on air guns. Other than Sarum’s issue with air blowing muck everywhere, we are dubious about using slightly wet air to try to shift greasy residues with muck bound into them. It does not sound good to us.

Don’t assume that a bag of seals is actually clean. They most likely need cleaning.

Much as plating or finishing process may be brilliant in “cleaning” a part, don’t assume that the plating tanks are clean. Sounds confusing! Just wash the parts when they come back.

Don’t let technicians assemble stuff wearing woollies or fleeces with synthetic fibres.

Sarum thinks the hydraulic nightmare is building stuff up with thousands of synthetic fibres, human hairs, fibres from cardboard and paper plus other assorted debris stuck under seals. It is the most depressing thing in the world as the job will leak and might as well be a scrapper. That is the default condition if you don’t take cleanliness seriously.

Some parts are surprisingly inherently filthy.

I think that rubber hose material is always suspect. Hose assemblies need cleaning through the bores.

And finally for flushing and condition monitoring.

 Sarum thinks you can achieve incredible improvements in reliability of hydraulic equipment  through flushing it in production or alternatively during commissioning. That is simply what we have found when it has been implemented on larger volume production. System builders very commonly include their high pressure and tank retun filers then run the system during commissioning, changing the filters for new elements after flushing.

What is involved in flushing? People will sell you a flushing rig, so you are buying in know how. We have just manufactured our own rigs. In principle, a gear or piston pump supplies oil to a high pressure filter, it is run through the job then back to tank via a return line filter. What are important features from our experience?

Flow has to be fast enough to produce turbulence in the flow passages. One of our units uses a three phase motor with an inverter to produce a variable speed unit. That is mighty useful. We would do this again despite the cost.

The filters must have clogging indicators. If a filter clogs and bursts, that is bad news.

The piece of hydraulic equipment has to be effectively flushed, which invariably means getting oil through all passages. On one job, we don’t fit two Koenig expanders before flushing and have a clever fixture with sealing washers to take the flows out of multiple exits from a manifold body.

Deskilling connection is important to speed things up. The moment it becomes a backbreaking chore, there is something wrong. The same job uses no threaded connectors for flushing but has a clever fixture with Destaco toggle clamps forcing the body up against some clever Sealjet sealing washers made for us by DMR seals

Productivity is money. This same unit flushes two bodies at one time by mirroring the fixture and feeding flow into a common block. It is brilliant!

Don’t get oil everywhere! Said rig as described has a decent milled base plate with channels and drains. Money well spent.

Minimess couplings are mighty useful for making sampling points. I like them. Stauff do similar couplings, as do other people.

We have already noted that any amount of flushing will not remove a lump of fibre stuck under an o ring during assembly. Flushing is good but don’t over sell it.

 Finally how clean is clean?

If you just flush for a certain time, you have no yardstick as to how clean it might be if you left the rig running for twice the time. We still send samples off to Hydrotecknik in their little bottles and get an excellent service. Remember, they have the kit to do more than just an ISO4406 analysis of counts at different sizes. Water content can be important for some people, as can the actual elements present in the sample, giving some clue as to what the source of contamination actually is.

We bought a Filtertecknik in-line cleanliness analyser as well which gives us our readings real time. You can measure your ISO counts and watch the fluid getting cleaner. There is a little bit of fiddling to set it up and don’t forget that like any instrument, it needs recalibrating. We leave it running on just one flushing rig and one grade of oil. Our biggest lesson in cleanliness measurement is it is risky to step away from what you know. Can you use our meter to measure cleanliness of water glycol? Our advice is to steer well clear.

Another option on instrumentation is a meter that will analyse a fixed sample similar to that which we send off for analysis. That could offer attractions, as it is immediate. MP Filtri do a unit, as I am sure do others.

A final question is how clean does your system need to be?

Chasing high levels of cleanliness isn’t a bad call if you have a flushing rig running and particularly if you have servo valves in a system or components intolerant of contamination. Whatever the system, why would you want to run it with particles floating about which will just accelerate the wear in pumps and actuators? However clean a system may be after flushing, once it is operating it will undoubtedly generate more contamination even at very fine levels, which you hope will be removed by filtration if this is in place. That does not answer how clean the system needs to be, although the guidelines provided in technical literature for various generic circuits always seem sensible. It is always worth pondering that catastrophic failure in a system can often be a result of dirty oil beyond comprehension, with filth that you can see by eye as opposed to a microscope.

And the future?

System cleanliness is a big deal. If more and more systems are equipped with small relatively inexpensive built in sensors that are transmitting results by cable or wirelessly to a controller, you are raising awareness on cleanliness. Where the gearbox of a Public Service Vehicle or train may today be equipped with a contamination sensor which sends results to a central control room and see potential failure looming at the very earliest stages, this technology will seep down into all sorts of systems.