About Stage 3
When troubleshooting hydraulic or pneumatic cylinders, a great deal of information can be gathered through careful inspection of the various components that have been disassembled and cleaned. The following is a guide to assist in performing or specifying the proper cylinder upgrades. Look for these conditions:
- Damaged Rods
- Damaged bores
- Improper design of glands and pistons
- Side loading conditions
- Improper seal fits
- Improper surface finishes
- Surface finish
The following is a more detailed description of each of these damage categories, together with suggested causes and possible solutions.
Damaged rods seem inevitable, but with some guidelines, much of that damage can be avoided. Damaged rods destroy seals and bearings.
|Cause of Rod Damage||Solution|
Damaged externally by failing materials.
Caution operators. Install protective shields.
Damaged externally by tools.
Caution maintenance personnel.
Scored externally by abrasives in environment.
Use sharp lip lubricated wiper with high abrasion resistance and long term memory.
Scored internally by contaminants under pressure.
Avoid contamination of fluid. Use sharp, positive rake lip designed polymer rod seals. Avoid mechanically activated (spring, O-Ring) seals.
Flaking of Chrome.
When re-chroming, proper surface preparation is critical. Check substrate for cleanliness.
Bent rods, broken rods.
Identify source of bending load and correct. Identify cause of binding. Check mounting and clevis arrangement. Check rod size to task involved.
Cylinder bore damage may have several causes; and, once damage has occurred, seal life and cylinder performance will diminish. Honing or re tubing may fix the effects, but care should be taken to correct the cause.
|Cause of Bore Damage||Solution|
Identify cause of contamination and correct. (See: Operating Conditions, Fluid Contamination). When using U-cups, use sharp, positive rake lip piston seals.
Foreign material trapped in cylinder.
Secure threaded parts. Check carefully upon reassembly for loose materials.
Metal to metal contact, piston to bore.
Use non metallic bearing bands.
Side loading will cause several cylinder problems but the effects are virtually always seal failure and equipment damage. Attention to bearing materials and fits will improve cylinder performance.
|Cause of Side Load Damage||Solution|
Poor original cylinder design i.e., stroke too long.
Add bearing area if possible. Remanufacture cylinder if necessary.
Poor bearing fits.
Correct to accepted standards.
Eccentric bearing fits.
Make bearing fits concentric to centreline.
Dissimilar bearing material on rod end and piston end causing uneven wear.
Use similar material on rod and piston ends. Non metallic, glass filled nylon bearing bands are preferred.
Misalignment or improper mounting and clevis arrangement.
Check for misalignment or mounting and clevis arrangement causing “built-in” side load.
Improper seal Fit
Seal grooves may be designed or machined poorly, causing a seal to be crushed or otherwise damaged. Redesigning grooves or more careful seal dimension selection can remedy these problems.
|Cause of Improper Fits||Solution|
Excessive extrusion gap clearance.
Maximum clearances vary with seal compounds, temperature and pressure. As cylinder wear increases, use of seals with higher maximum running clearance capabilities will increase performance. Use composite seal with molded in back up ring.
Radial dimension of seal groove too narrow or too wide.
Seal groove radial spaces variation should not exceed design tolerances. Variation is dependent upon pressure.
Seal groove axial dimension too narrow or too wide.
Axial clearances vary for automatic U-cups, cap seals, piston rings, etc. Consult seal manufacturers design guides.
Improper Surface Finish
Hydraulic and pneumatic seals must create a positive seal both statically and dynamically. A static surface finish too rough can cause leakage. Dynamic surface finishes can also be too rough or too smooth.
|Cause of Improper Finish||Solution|
Incorrect selection or specification of rod or bore finish relative to application.
Refer to seal manufacturer’s guidelines for surface finish recommendations. Consider fluid lubricity and surface speed. Use softer seal material like our 85A Blue Polymer.
Static seal groove finish too rough due to dull machine tool or improper machining practices.
Use sharp machine tools and corrects speeds. Inspect groove for flaws. Use softer seal material like our 85A Blue Polymer.
Pitted grooves caused by corrosive nature of fluid or environment.
Replace or repair with protective plating such as nickel.
Improper Gland and Piston Design
Hydraulic and pneumatic cylinder designs and materials may not be adequate for a particular application. Once flaw is identified, upgrades can be performed to improve performance.
Inadequate construction materials.
Materials used should be resistant to pitting and strong enough to meet load bearing demands and stresses for a particular application. Use high strength, non metallic wear surfaces.
Inadequate space allocation for seals, wipers and bearings.
Whenever necessary to increase service life, increase axial and/or radial dimensions for seal, wiper and bearing area.
Redesign glands and pistons. Simplicity and strength are generally preferred over complex designs, e.g. one piece pistons are generally preferred over three piece designs.
Design variations within cylinder type.
Standardize on one gland and piston design for each cylinder type. Assures fit of approved seal kit, reduces parts inventories and controls evaluation of cylinder life.