A guide to shaft seals
By Joseph L. Foszcz, PE/CPE, Senior Editor, Plant Engineering Magazine -- Plant Engineering, 3/1/2001
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
|
Symptom |
Cause |
Cure |
|---|---|---|
|
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 |
|
Symptom |
Cause |
Cure |
|---|---|---|
|
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 |
|
Symptom |
Possible 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 |
|
Problem |
Causes |
Corrective 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 |
