Can I pipe up my hydraulic system in microbore -03 or -02 hoses for ultra compact? Learn more on using microbore hoses to make a very neat job, produce a masterpiece or even open up a new hydraulic application.

Photograph of a silver convertible car with its roof partially retracted, showcasing the retractable hardtop mechanism. Next to the car, various components of the retractable roof system, including hydraulic cylinders and cables, are displayed on a white background.

A hydraulic sunroof on this Mercedes SLK is a masterpiece of miniature hydraulics. Can your project use the same microbore hoses for a winning solution? Image Cayman Autos for hood and Top Hydraulics for their cylinder/microbore hose packages.

Connecting up the right system in miniature hoses can be a winner especially if you are using miniature pumps, cylinders and actuators. We all know just how neat the hydraulics are on an automotive soft top convertible. That was a real groundbreaking product in the automotive industry, although the idea had been around a long time. The MB SLK sunroof was a whole different proposition. Miniature mass market micro hydraulics probably made that lovely car and all the other roofs happen. So, let’s translate the same amazing space envelope and neatness to our industrial, yacht, marine, AFV or vehicle application.   We are going to look at the decisions you need to take and the various hoses in the marketplace. We have used most of them over 40 years, so have some expertise. There are a few big considerations from our perspective. No AI on this article.

A miniature hydraulic system using microbore hoses has extra complications over normal working around a massive range of catalogue kit running right up to 700 bar for “mainstream” hydraulics.

We think you need to sort out three questions.. Clearly, those three questions are all linked and you can’t design around a pressure where you can’t buy the hoses. Anyway, designers have to start somewhere. This is our thought process.

What cylinders or actuators can I use and at what pressures to achieve my objectives?

Will miniature microbore hoses work for me with the flows needed both on the pressure feed, return lines and also suction lines if those are part of the micro hydraulic system?

What hoses can I specify from the marketplace to meet my needs on pressure, chemical compatibility, cost and availability?

Let’s look at each question in turn.

What cylinders or actuators can I use and at what pressures to achieve my objectives and what is the system pressure for your microbore hose system.

Just like any hydraulic system, you are specifying a pump, cylinders or actuators and maybe a control valve. The difficulty on ultra-compact systems using microbore hoses is that your pool of catalogue components is much smaller. Here we are purely looking at medium and higher pressure microbore hose systems with smaller actuators. We deliberately aren’t going down the route of looking at very low pressure hydraulics at say 10 bar. That is quite specialist and we have a paper on the subject. We are running our calculations on pressures of up to 200, 300 or 400 bar as a typical staring point. You need to source whatever type of pump you require then sort out cylinders. That is where we would start in today’s discussion.

The miniature hydraulic crew seat pump and pull cylinder is one of our OEM products. We designed and manufactured this compact 350 bar pull cylinder for the exact application

We have used miniature cylinders from catalogue ranges like Enerpac pancake cylinders or specialist “tooling” ranges from Roemheld and Enerpac, amongst others. On other projects, we have had to design and manufacturer our own miniature cylinder, for example a very small PULL cylinder with 14mm rod, 20mm bore and 75mm stroke for a very small hydraulic seat mechanism to 350 bar on an AFV. Be prepared to make your own or buy custom miniature cylinders. Your buckling load on the rod will be an issue on spindly small diameter rods/long stroke/high pressure miniature cylinders. That is an issue on micro systems. Can I specify a “larger” catalogue cylinder and pipe it up with microbore hoses. Yes, fine, but be aware that you may be passing a lot of oil through small hoses and the cylinder is slow to extend and slow to retract. The design might look far too bulky.  Possibly ridiculous. Do the simple calculations of force = pressure x area to provide working forces. Then Piston area x stroke for the swept volume of a cylinder. That is your starting point.

So, in summary, you must have the cylinders that will achieve your load and stroke objectives. You will know the target force required for the job, can calculate the cylinder force that can be generated and  the swept volumes for the fluid as the cylinder extends/retracts. You can then get a feel for target cycle times for the extend and retract cycles.

Will miniature microbore hoses work for me with the flows required on the pressure feed, return lines and suction lines if those are part of the micro system? Are the size of hose and the run of hose on the pressure line compatible with your hydraulic functionality?

This is our second question. We would say the starting point is hose bore and run.

It is stating the obvious to say that hoses cause pressure drops and hydraulic systems don’t work effectively with excessive drops and also wasted energy. As hand pump people, we are very sensitive to them. On a pressure line with very small hand pump flows, you can probably live with small bore hoses and quite long runs. Not so with a powered pump if the flow rates are much higher. Pick a hydraulic fluid that is relatively viscous or operating conditions that involve cold weather and viscosity becomes a key consideration. A cylinder might be too slow to extend if you are putting too much viscous fluid through too small a hose. Get a feel for the fluid you are using by looking at Lee products excellent viscosity v temperature graph. We have really rated these for years. We’ll talk about Pressure as our next issue.

Start with the sizing on the pressure feed lines then consider return lines where fluid is being returned to tank at low pressure. It is useless finding that the retract speed on a ram is totally unacceptable because the hose is a long run on small bore and the fluid viscous because of the cold. You have nowhere to go other than fitting a larger hose. That isn’t our objective here.

Should I be careful on hose bores on suction lines? Have we enough suction line bore for our system to work?

Yes, on suction lines, be very careful on hose bore and run length. These are the feeds from tank to a pump. This question is very close to the first point about getting enough fluid thought your microbore hoses to do the job. Basically, we are asking whether our microbore hoses are going to work on the suction side? Like all hand pumps, our Micropac units are sensitive, especially small displacement units. Our two speed hand pumps like our MD or PPS use a clever technical fix to charge the small displacement high pressure stage. Small displacement gear or piston pumps have similar issues with relatively restricted inlet paths, whether caused by using small bore hoses and long runs or indeed a technical issue like a squashed hose or blocked strainer. Power pumps often use a larger displacement low pressure pump in tandem, incorporate a low pressure georotor pump or a split flow pump. You are feeding the pump inlet with low pressure fluid to assist it. If the suction line in genuinely too small or constricted, this fix might not really address the root cause. You need larger bore hose.

To check that your hoses are large enough bore for the flows you are putting through them on the pressure and suction lines, Parker are a good starting point with this excellent guide. Plus keep that eye on temperature ranges. We’ve found this more and more important in a globalised world where you kit might end up being used on an oilfield in Canada with extreme cold experienced.

Maybe some other good advice is trying to keep the reservoir close to the pump or even build it in. Tank top designs or the industry standard “end head” are very neat. Remote reservoirs are often necessary but maybe don’t make them too remote or fight too great a head.

When would we consider hose materials for chemical compatibility? 

This is quite important, as it might flag up an issue. Here we are talking the generalities of materials rather than the different types of hose.

We will move on to types when we consider small bore hoses. Picking the right hose material or at least what is unsuitable might almost be more important than pressure rating. If you are running on mineral oil, life is quite easy. Punishing environments would probably move you from plated carbon steel to stainless steel fittings. For water hydraulics, again a range of materials is available, invariably moving to stainless steel end fittings. Our tip is to keep an eye open for warning signs on restricted supplies of certain end fittings, either as long lead times or “special order.” Your mineral oil hose might use a rubber liner with steel wire reinforcement or polyamide tube with fibre strengthening. A ptfe hose with stainless overbraid is another option with excellent compatibility. Your polyamide or ptfe liner in contact with your fluid would also have wide chemical compatibility. A liner made from elastomer would need selecting according to the fluid. A water glycol based brake fluid wouldn’t use a nitrile rubber hose. Search manufacturer tables to select your materials. The Parker guide is good, but you will have a favourite. Do also take into account the outer sheath of a hose. There is little point in using a very chemically inert liner only to find that the outer sheath is rotted by the fluid.

What hoses can I specify from the marketplace to meet my needs on pressure rating, chemical compatibility, cost and availability?

Let’s look at the marketplace. First, just check there is an opportunity or reason to look at microbore hoses.

Is it worth bothering with a miniature hydraulic system and microbore hoses?

We would say yes in the right application. But not just for the sake of doing it.

If you have two or three -04 size hoses running through an installation and compare them with two or three microbore, neatly routed, there is a massive reduction in size. It all depends on the application. You wouldn’t bother if there is no need. But get it right, and suddenly a hydraulic system becomes a transformation and a winning solution. Designers make the call.

What do we mean by small bore hoses? How small?

For over 40 years, we have sold customers -04 bore hose which is a nice size in conjunction with our kit. It coils nicely, isn’t vulnerable to damage and mates well with our pumps. Customers would then be offered whatever end fitting on the free end to suit their application. Contrast this with using a -06 hose or monster -08 hose (by our standard) and having to coil that on a Micropac MP filler unit or DUO 2 wheel cart. Other applications with big pumps might demand even larger bore hose.

In the context of our miniature hydraulic systems, we are looking at -03 bore hose or dropping to -02 bore if we can live with the smaller flows. That is a 4.7mm/3/16 inch or 3.2mm/0.125 inch bore on -02. That translates to 9.8mm/0.38 inch external diameter on the -03 and 7mm/0.28 inch external diameter on the -02. That is very small. Our standard -04 rubber hose with a single steel reinforcement is 13.4mm/0.53 inch, which is substantially larger. For the right job, you are talking low mass, compact and neat if you go -02 or -03 microbore.

Is -02 hose the same as DN02?

The short answer is no. The long answer; be very careful when specifying hose sizing.

The “Dash” hose sizing system was developed by the Society of Automotive Engineers (SAE), who developed the standard in North America to provide a shorthand way of identifying hose sizes without writing out the full fractional size. Their dash sizing system is based on 1/16’s of an inch. Therefore, a “-4” or “Dash 4” hose is 4/16 of an inch, or 1/4” bore (internal diameter). A -2 hose is therefore 1/8” or 3.2mm.

What then is the “DN” notation for hose sizing? DN stands for Diamètre Nominal (French for Nominal Diameter). This is the metric standard used globally and is defined by ISO 4397. The DN number just refers to the I/D of the hose in millimetres. A DN02 hose is therefore a hose with a 2mm bore.

Some of these hoses cross over and are nearly identical. A -3 hose and a DN5 hose are close enough to basically be the same product.

Another hurdle is making sure that the Dash size for the threaded connector is also specified. A Dash 4 hose doesn’t always use a dash 4 thread!

Our take is that the dash size convention is mighty useful and does focus the mind at early stage design. For example, our catalogue Micropac hoses are dash 4 with G1/4 Gas female swivels each end. The most common -4 male male adaptors in the end 1/4bsp or ¼ npt. -4jic or -6jic is possible. Step up adaptors take to -6 like 3/8bsp or 3/8npt.  -8 takes you to 1/2bsp or 1/2NPT. This is easy configuration in our world.

We have this handy web sheet for looking up the thread and hose dash sizes. The threads listed here are for JIC threads. Sorry that the 1/8 inch / -2 line isn’t included. That does flag up that -2 is quite specialist.

What size is pushbike hydraulic brake hose?

Let’s flag up these hoses. They are relevant to our discussion. Shimano hoses are 2.1mm or 2.4mm bore and 5mm external diameter. That is very compact. It all comes down to construction, durability and for this application, a desire to cut out mass. The construction is using braiding to reinforce a core, so a variant on the “thermoplastic” construction that we will talk about as one of our options.

What types of microbore hoses are available and to what pressures?

We have talked about being aware of your fluid and what hose material isn’t compatible chemically. We’ve also picked up on needing a certain bore for your hydraulics to work. Let’s run through your options, as we see them.

Mainstream rubber hydraulic hose with steel overbraid.

Wire reinforced rubber hose solves the bulk of hydraulic hose needs for mineral oil and water. There are limitations on very small bore hoses. Image courtesy of Sinopulse.

A -03 size with 4.7mm/3/16 inch bore is commonly offered. The external diameter is up to 10mm. If that works for you that is fine. For our miniature hydraulic applications like high performance sailing dinghies, medical hydraulics, sunroofs or hydraulic crew seats, you would opt for smaller profile hoses with no particular downside apart from maybe cost. You are talking modest flow rates.

Small bore thermoplastic hoses.

A typical Thermoplastic hose. Poylamide/PU line, textile reinforcement and PU or similar outer sheath to protect the reinforcement. A good hose option, subject to odd pluses and minuses.

These are one very good option. Thermoplastic hoses are big business. Traditionally, Enerpac and similar 700 bar hydraulics have run on thermoplastic rather than the heavier multispiral rubber hose using a rubber liner then multiple winds of steel wire to achieve burst resistance.

Thermoplastic hoses use a plastic hose liner (commonly polyamide/nylon or polyurethane) then a textile reinforcement. The textile might have a complex form to provide better bend resistance, as traditionally people worried that these hoses had a tendency to kink if taken though a tight radius or crush in use. Our impression is that the more sophisticated textile layers have made these hoses more durable. All these hoses need a sleeve in polyurethane or similar to protect the textile layer in use. Keep in mind that the “textile” layer could be Kevlar or similar aramid fibre, for higher performance high pressure hoses.

On our -02 and -03 sizes, we would look at Parker 540-02 with a polyamide liner, textile reinforcement and a polyurethane sleeve Those run to 207 bar. We often run our microhydraulics at 300 or 400 bar. A good choice is the basic German Spradow test point hose that runs to 407 bar with the standard textile reinforcement or 630 bar with aramid fibre reinforcement. That higher pressure is very handy. This hose is only 5mm external diameter. Our friends, Hydrotechnik offer hose assemblies in a similar material. We have used these and like them.

For years, we have been massive fans of the small bore Polyflex hoses. We also use their larger bore variations on our PSP two speed hand pump and our PTR-M air driven skid up to 1000 bar. They are very strong on ultra high pressure hoses. The small bore hoses such as the 2040N-02 and 2040N-03 -02 and -03 series take a thermoplastic hose and for some reason seem amazingly flexible with an external diameter at only 7mm. The working pressures are 340 bar on the 3mm and 350 bar on the 2mm. For us, the big win is the lovely flexibility with a tight bend radius. These were used for decades for a tortuous path through the frame on our innovative crew seat hydraulics. Polyflex started as an innovative German company and were bought by Parker a few years ago. Two points. They always were a specialist product in the market place and our buyer has been told on some enquiries that odd variants on Polyflex are no longer available in the UK market. That is onerous to buy from the USA. Feel free to correct us on this. Some fittings may be on long lead time and high price as well. That can be a major problem. So, be careful on specifying a end fitting until you have checked it is available.  Finally, our impression is that these hose assemblies are commonly bought in by your hose supplier as an assembly from the factory nowadays. They no longer buy hose on the reel, fittings then assemble. Maybe that is increasingly common across the whole industry. Somebody please put us right on our favourite Polyflex hoses.

PTFE with stainless overbraid.

Stainless steel overbraided ptfe hose. A great option for special needs, including small bores. Images courtesy Duncan Rogers.

For -04 size hose, PTFE with a stainless overbraid can be a winner as the material is incredibly inert over a range of fluids. The cost is higher, and also be cautious about somebody kinking the material in use. Remember that there are smooth bore and convoluted liners, with the latter being very good on bend radius. Wind back to our microbore -02 and -03 sizes and PTFE becomes a very good option. This type of hose is very widely used in motorsport and automotive. Look at Goodridge or HEL. HEL will offer a very good product running to 290 bar. Commercially, HEL are keen as a genuine OEM trade supplier.

Nylon or polyurethane tube.

Plain nylon/polyamide tube. Incredibly cheap. Low pressure. Great in the right application but not for this discussion. Image by PCL

There are some hydraulics built around being able to use plain tube with no reinforcement. That is “low pressure hydraulics” mentioned at the beginning of this article. It has a useful place, but not in this discussion. This material is maybe limited to 20 bar. We are thinking of Bennet motor boat trim tab hydraulics or maybe even domestic fire misting systems working at low pressures. This is a thought, but we are looking at higher pressures here, as we said initially. Note that a thermoplastic hose is often based upon a polyamide (nylon) tube with a textile reinforcement and an outer sheath to protect the structure from damage.

Motorcycle hoses and motorsport products

HEL are one of the motorsport suppliers who we would look to on microbore hoses.

We would be happy specifying -02 or -03 hoses from suppliers of motorcycle or automotive hoses like Goodridge, AN Motorsport or HEL. Remember that water glycol brake fluid requires a different hose liner to mineral oil unless you opt for ptfe or polyamide liners. So that suggests that ptfe and thermoplastic hoses are more useful than nitrile rubber for a wide range of fluids. That’s a basic point. There are two other big plus in going this bike/motorsport route. For us, the range of fittings offered meshes with our own needs for equivalents to US MS and AN standards, as well as BSP. Fitting materials include anodised alloy, plated carbon steel or stainless steel.  Be specific on stainless grades. If, like us, marine grade 316 stainless is your requirement, do specify it. HEL hoses seem to be commonly available with 303 and 304 grade fittings, not 316. One great port of call is the Goodridge catalogue if you are a designer or specifying a hose of this type. For motorsport and motorcycle applications, quality standards are very high, as there is a risk to human life if the hose fails.

One over riding point here is to check fluid compatibility. Brake fluids don’t like nitrile rubber. If you are on ptfe hose or a polyamide liner, you have a more universal compatibility. Ask the question.

Are motorsport bits expensive? As a company, we have always been cautious of the massive costs that are routine in the motor racing business. We would say that now the “motorsport” sector is so large, there are suppliers who are very realistic in supplying a wider mainstream market without trumpeting that they supply various F1 teams.

Pushbike hoses

Shimano and other cycling hoses are a massive application. Are they relevant here? Image by Shimano

We have briefly discussed Shimano and similar. Could you or should you use pushbike hoses on a microbore industrial application? Pushbike brakes use mineral oil, so the material is compatible with oil. The burst pressures are up at 400 bar with working pressures typically (say) 250 bar for a minute or two. The hose material is probably optimised for its application. How much mishandling would it stand? We aren’t sure but think we would run with automotive hoses or industrial small bore products. Tell us we are wrong and that savvy OEM’s do use pushbike hose.

“Test point “ hoses.

Hydrotechnik test point hoses offer a great microbore solution. Image Hydrotechnik.

Test point hoses are the small bore hoses supplied by people like Stauff and Hydrotechnik for instrumentation people to connect their gauges and instruments into a system. One end commonly uses a “test point” like Hydrotechnik’s “Minimess” connector. We standardised on these and think they are a great product. Other people make similar things. These test point hoses seem to be the same as thermoplastic microbore hoses with the liner, textile reinforcement and a sheath. In years gone by, we found some test point hoses very prone to kinking. We were probably asking too much. Now we would be specifying microbore hoses maybe with a more robust reinforcement and possibly a mm larger on diameter. Maybe nowadays they are the same, but we always need something that is fairly robust and good on bend radii. All we can report is that having used little test point hoses years ago that seemed to kink, our experience with the Hydrotechnik microbore test point hoses often fitted with minimess couplings has been wonderful. They are a great product.

What Micropac pumps can Sarum Hydraulics supply for Microbore hoses?

Our Micropac YM pump offers a miniature single lever hand pump to 400 bar. A great solution for the right application.

Our Micropac MC pumps offer a very small space envelope 400 bar. Applications include testing, calibration, actuation and even charging refrigeration with PAG fluid. A great solution for the right application.

Hand pumps tend to be small flow rate anyway, so probably any of our models would run happily on -02 and -03 microbore hose systems and miniature cylinders. Ultra neat and compact micro hydraulics sounds to us like smaller space envelope pumps, so look at the MC or YM series as a starting point or ask us. The YM was designed for miniature marine and defence microsystems, with a very small space envelope and a single lever control with pump down to generate pressure and pull back to release. That is an invaluable feature for some applications. The YM can be integrated onto a very small panel with a pressure gauge and four way directional or distribution valve if required. Apart from our single miniature pull cylinder, we can’t supply any catalogue miniature cylinders.

What Micropac miniature four way directional control valves can Sarum Hydraulics supply?

A big problem with miniature hydraulics using microbore hoses is that the related parts are still massive mainstream catalogue units. Four way directional valves are an issue. We supply a truly minute 316 stainless valve at a reasonable price. Contact us.

Ask the experts in manual hydraulics.

After over 40 years, we are the experts on manual hydraulics. Contact us if you need any help in creating a miniature hydraulic system using our ultra compact pumps.