Tips on se­lect­ing and re­plac­ing hy­draulic hose assem­blies

Australian Forests and Timber - - Maintenance -

The im­por­tance of se­lect­ing or re­plac­ing hy­draulic hoses and fit­tings of­ten gets over­looked un­til some­thing goes wrong. En­gi­neers must con­sider sev­eral fac­tors be­fore choos­ing hy­draulic hose assem­blies, to en­sure that th­ese crit­i­cal parts are cor­rect and fit for pur­pose. Here, Gates pro­vides some key tips to bear in mind when you’re putting to­gether your hy­draulic as­sem­bly.

Hose and fit­ting ba­sics

Most hoses have three func­tional lay­ers – the tube, re­in­force­ments, and the cover. Hoses are built and tested ac­cord­ing to in­dus­try spec­i­fi­ca­tions such as SAE and EN and each com­po­nent is de­signed to re­sist the pres­sures it will en­counter, like abra­sion, per­me­ation, bend­ing and tem­per­a­ture changes. En­gi­neers should be fa­mil­iar with spec­i­fi­ca­tions re­lated to equip­ment the hoses they de­sign will go on. They pro­vide guide­lines for di­men­sions, ma­te­rial prop­er­ties, and min­i­mum per­for­mance char­ac­ter­is­tics for ma­jor types of hose and fit­tings.

Fit­tings make leak­free con­nec­tions to the hose (fit­ting end) and at­tach the hose to dif­fer­ent com­po­nents in the hy­draulic sys­tem (ter­mi­na­tion end.) Most fit­tings have two parts, a stem and fer­rule. The stem con­veys the fluid, at­taches to the hose, and pro­vide a seal be­tween the stem and hose. The fer­rule at­taches to the re­in­force­ments and serves as a “weather seal” to pro­tect the hose and fit­ting from the en­vi­ron­ment. Both are typ­i­cally made of car­bon or stain­less steel and are some­times plated or coated to im­prove chem­i­cal or cor­ro­sion re­sis­tance.

The ter­mi­na­tion end forms a leak-free con­nec­tion be­tween the hose and a port, adapter, or another hose. There are many dif­fer­ent types of end con­nec­tions spec­i­fied by in­dus­try stan­dards (such as SAE J516). Know­ing how they fit to­gether is key to se­lect­ing the best ter­mi­na­tion for an ap­pli­ca­tion.

Defin­ing re­quire­ments

Size:

A hose’s two crit­i­cal di­men­sions are in­side di­am­e­ter (ID) and over­all length (OAL.) The ID must be sized to keep fluid ve­loc­i­ties within rec­om­mended ranges. If the ID is too large or too small, it can change fluid flow and hurt ma­chine per­for­mance by caus­ing ero­sion, ex­ces­sive pres­sure drop and power loss, too much tur­bu­lence (heat gen­er­a­tion), pump cav­i­ta­tion, and noise.

The OAL must be sized to pro­vide proper rout­ing. Too short and it puts ex­ces­sive stress on the hose and fit­ting and pre­vents them from bend­ing and stretch­ing due to pres­sure im­pulses. If the hoses are too long, they can rub against each other or nearby com­po­nents, or get caught on some­thing.

When re­plac­ing a hose, you can de­ter­mine the ID by check­ing the la­bel. If it has been painted over or worn off, do not use the hose OD to iden­tify the hose’s ID. Hose ODs vary ac­cord­ing to con­struc­tion and man­u­fac­turer and have no di­rect cor­re­la­tion to IDs. A bet­ter op­tion is to cut the hose and mea­sure the in­side di­am­e­ter. Re­mem­ber to record the over­all length and fit­ting ori­en­ta­tion be­fore cut­ting the hose.

Tem­per­a­ture:

It does not take much ex­tra heat to shorten a hose’s ser­vice life. A good rule of thumb says that for every 10°C over a hose’s max­i­mum tem­per­a­ture rat­ing, it short­ens the hose’s life by 50%.

Be sure to choose hose with an up­per limit well above the ma­chine’s op­er­at­ing tem­per­a­ture.

Hoses’ tem­per­a­ture rat­ings, both min­i­mum and max­i­mum, are usu­ally listed in agency spec­i­fi­ca­tions, prod­uct cat­a­logs, or tech­ni­cal pages. Most rat­ings are for nor­mal ser­vice con­di­tions, but spe­cial rat­ings are some­times listed for in­ter­mit­tent ser­vice. Plas­tic hose pres­sure rat­ings are of­ten re­duced by a tem­per­a­ture fac­tor. Ap­pli­ca­tion: En­gi­neers need to doc­u­ment any­thing spe­cial the ma­chine they are de­sign­ing is ex­pected to do, in­clud­ing safety and en­vi­ron­men­tal risks, and ex­treme con­di­tions the hose will be ex­pected to han­dle.

If re­place­ment hoses are be­ing made, ask some ques­tions rather than just du­pli­cat­ing the orig­i­nals. How did the orig­i­nal hose fail? Does it show signs of cover abra­sion or tem­per­a­ture cracks? Does the ma­chine gen­er­ate pres­sure surges, or is it more of a static ap­pli­ca­tion? Make sure to find a re­place­ment hose that best matches the ap­pli­ca­tion re­quire­ments, not just the orig­i­nal part.

It’s also a good idea to brush-up on new hose tech­nolo­gies. Over the past sev­eral years, some man­u­fac­tur­ers have

de­vel­oped hoses and fit­tings that far ex­ceed the per­for­mance and con­struc­tion ca­pa­bil­i­ties of SAE spec­i­fi­ca­tions. Hoses that meet or ex­ceed SAE per­for­mance specs but are smaller than stan­dard hose can be good choices.

Ma­te­rial:

The most crit­i­cal hose and fit­ting ma­te­rial con­sid­er­a­tion is chem­i­cal re­sis­tance, so con­firm what flu­ids will run through the hose and what chem­i­cals will touch the fit­ting bore, fer­rule and ter­mi­na­tion. Al­ways check the fluid’s com­pat­i­bil­ity not only with the tube, but with the re­in­force­ments, cover, fit­tings and seals. Pres­sure:

Work­ing and burst pres­sures are the two most com­mon pres­sure rat­ings for hoses. Work­ing pres­sure helps se­lect the cor­rect hose based on the hy­draulic pres­sure. Sys­tem pres­sures should never ex­ceed a hose’s work­ing pres­sure. Burst pres­sure is the max­i­mum pres­sure a hose can take be­fore cat­a­stroph­i­cally rup­tur­ing. It also pro­vides an es­ti­mate of de­sign or safety fac­tor. Burst pres­sure is usu­ally four times the work­ing pres­sure for most hoses.

It is crit­i­cal to know the full range of hy­draulic pres­sures when choos­ing hose, in­clud­ing pres­sure spikes.

Hoses‘ work­ing pres­sures should be greater than or equal to the high­est sys­tem pres­sure (in­clud­ing spikes.) Sub­ject­ing hose to pres­sures above its rated work­ing pres­sure re­duces its ser­vice life and in­creases the chances it will fail un­ex­pect­edly.

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