A guide to shaft seals

Machines require rotating shaft seals to retain lubricants and prevent foreign particles from entering sealed cavities, which damage machinery and lead to premature failure.

03/01/2001


Machines require rotating shaft seals to retain lubricants and prevent foreign particles from entering sealed cavities, which damage machinery and lead to premature failure. Application conditions can vary considerably, and many seal designs have evolved to satisfy these conditions. Some applications tolerate a small amount of leakage, while others cannot allow any leakage. In general, seal complexity and cost increase as the need for zero leakage increases.

Shaft seals make up two groups: contacting and noncontacting. Contacting seals make direct contact between the sealing component and the rotating shaft. Because there is rubbing contact, leakage is minimal and friction and seal wear occur.

The sealing element in noncontacting seals allows a certain amount of leakage. It is controlled by clearance gaps. Because there is no rubbing contact, leakage is higher, but seal friction and wear are eliminated.

Contacting

There are three types of contacting seals.

Mechanical face seals prevent leakage in applications that exceed the capabilities of elastomeric radial lip and packing designs. They provide long life without shaft wear. Shaft finish, hardness, and material are not critical. The seal can handle a wide variety of fluids, pressures up to 3000 psi, speeds to 50,000 rpm, and temperatures from -425-1200 F. This type of seal has become popular on pumps where leakage is not allowed.

Radial lip seals are primarily used to retain lubricants and exclude contaminants. The seals function at temperatures from 60-400 F. Advantages include low cost, small space requirements, and simple installation.

Packings are not designed to produce a leak-free seal. They are tightened enough to allow a minimum, but positive, leakage. The leakage is intended to lubricate the packing material. The modest leakage reduces friction and wear. Packing material is supplied in many shapes, including impregnated yarn, continuous strands, square-sections, and interlocking shapes. This seal is generally used on pumps.

Noncontacting

Four types of noncontacting seals are available.

Bushing seals are classified as fixed, floating, balanced, and floating ring. The bushing seal is a close-clearance version arranged stationary to the housing or floating with the shaft. The fixed bushing is a sleeve attached to a housing, surrounding a rotating shaft with a relatively close clearance.

Floating bushing seals follow shaft gyrations and have closer tolerances than fixed bushings. Balanced bushing seals eliminate or reduce spring forces and still retain the advantages of a floating bushing seal. Floating ring seals overcome the pressure and alignment problems of single floating bushings by splitting the bushing into several rings. This seal is only suited for sealing liquids.

Labyrinth seals are used mainly in high-speed applications where relatively high leakage rates can be tolerated and simplicity is necessary. A properly designed seal does not require lubrication or maintenance. If wear occurs, the only damage is an increase in leakage.

Visco seals have grooves or screw threads machined on the shaft or in the housing to provide a positive pumping action. Viscosity of the pumped fluid in the clearance gap produces the seal effect. These seals function at certain minimal speeds. For low shaft rotation, a secondary-sealing device, such as a lip seal, must be provided. This type of seal is used where liquids are continuously transferred.

Magnetic seals use a colloidal suspension of magnetic particles, focused by permanent magnets, to create a seal. In addition to providing a nearly perfect seal with negligible wear or friction, they are tolerant of shaft runout. They can be used at speeds up to 120,000 rpm, temperatures up to 400 F, and pressures of 7 psi/stage. Seals are used primarily with gases and exclude moisture, mist, and fine solids.

&HEADLINE>Rotating shaft seals&/HEADLINE>

&BYLINE>Joseph L. Foszcz&/BYLINE>


&TEXT>

Contacting seals

Mechanical face

Radial lip

Packing

Noncontacting seals

Bushing

Labyrinth

Visco

Magnetic

Troubleshooting mechanical face seals

SymptomCauseCure


Narrow wear pattern


Excessive pressure


Reduce pressure or change to higher pressure-rated seal


Wide wear pattern


Cocked stationary face Pump or pipe misalignment Pump cavitation or vibration


Clean and reassemble seal Realign pump Check system piping


No wear pattern


Rotary seal interference


Disassemble and remove obstruction


Intermittent wear pattern


Distortion of stationary face


Check stuffing box face for flatness Tighten gland bolts evenly Check seal faces for flatness


Uneven wear pattern


Pump/motor misalignment


Realign pump and motor


Heat checking


Seal faces overheat


Add flushing to seal faces Change seal design or material


Cracked hard face


Thermal or mechanical shock


Change seal material Handle seals with care


Chipped edges


Fluid vaporization Pump cavitation


Properly flush seal Check system piping


Flaking and peeling


Defective coating Chemical attack


Switch to solid seal material Change seal material


Deep wear on hard face


Fluid is abrasive or crystallizes


Flush to remove abrasive products Add heat to inhibit crystallizing


Troubleshooting radial lip seals

SymptomCauseCure


Leakage


Nicks, cuts, or tears in seal lip


Remove burrs and sharp edges from shaft Use mounting tool to protect seal lip from keyways, sharp shoulders, and splines Handle with care


Scratches or nicks on shaft surface


Finish shaft to 20 (omega)in. rms Protect shaft after finishing


Lead marks on shaft


Plunge grind shaft surface


Shaft whip or runout


Locate seal close to bearing


Cocked shaft


Use correct mounting tool and procedure


Damaged spring


Use correct mounting tool and procedure Handle seals with care


Damaged case


Use correct mounting tool and procedure Handle seals with care


Paint on shaft or seal


Mask seal and shaft before painting


Turned under lip


Rough chamfer


Machine chamfer to 32 min. and blend into shaft surface


Chamfer angle


Check for correct lead angle


Careless handling


Use correct mounting tool and procedure


Excessive lip wear


Pressure


Provide vents in housing


Rough shaft


Finish shaft to 20 (omega)in. rms


Dry running


Provide proper lubrication for seal


Element hardening or cracking


Temperature


Reduce sump temperature Upgrade seal material Provide proper lubrication for seal


Shaft wear


Abrasives


Use dust lip in dirty atmospheres Harden shaft to Rc 30 minimum Change oil frequently to remove dirt Install wear sleeve


Troubleshooting packings

SymptomPossible causes/actions


No leakage


Incorrect installation- negative stuffing box pressure requiring lantern ring Slight overtightening- follow installation procedure


Excessive leakage


Wrong packing Incorrect installation procedure Shaft runout excessive


Packing reduced under horizontal shaft


Shaft misaligned with stuffing box Worn bearings- packing acting as bearing


Packing reduced over horizontal shaft


Shaft misaligned with stuffing box Worn bearings


Whole or part of end ring missing


Excessive clearance between shaft and stuffing box neck or gland follower, allowing extrusion


Wear on outside of one or more rings


Rings rotating with shaft, packing section too small


Rings next to gland follower worn, others all right


Incorrect installation procedure, gland follower overtightened


Ring IDs burnt, dried, or charred, remaining material all right


Incorrect packing selection for temperature or speed Inadequate lubrication


Innermost ring deteriorated


Incorrect packing selection, check compatibility with product


Packing seizure on shaft after shutdown


Product crystallizing or solidifying in packing bore through inadequate lubrication or lack of cooling or heating


Troubleshooting noncontacting seals

ProblemCausesCorrective action


Excessive leakage at startup


Excessive radial clearance


Decrease radial clearance


Excessive pressure


Improve or add vent


Excessive lubricant fill


Decrease fill or increase cavity


Excessive temperature


Decrease fill or increase cavity


Excessive vibration


Increase lube cavity


Excessive end play


Increase lube cavity


Radial contact


Fill scratches and voids


Bypass leakage


Replace damaged secondary seals


Excessive leakage after a period of time


Loss of lubricant viscosity


Improve lubricant


Excessive relubrication


Decrease lubrication


Contaminant ingress


Add contaminant shield


Loss of magnetic fluid


Replace magnetic fluid


Water ingress


Static leakage


Add static seal


Partial vacuum


Clean or add vent


No lube leakage


Increase lube fill


Excessive wet environment


Add water shield


Dirt ingress


Excessive radial clearance


Decrease radial clearance


Excessive dirty environment


Add dirt seal


Partial vacuum


Clean or add vent


No lube leakage


Increase lube fill





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