Some thoughts on shaft seal problems in rotary screw airends

Rotary screw airend manufacturers have used many types of shaft seals over the years. In most early airend designs, there were two distinct categories of shaft seals:   


  1. Mechanical seals were commonly designed with a carbon rotating element, bonded to the shaft with a buna-n or viton boot, sealing to a steel or ceramic stationary seat in the housing. Compressors with total closure (air tight) air inlet valves typically used these relatively expensive seals because they could withstand full system pressure during shutdown.   

  2. Lip seals were typically made of a high nitrile buna-n or viton rubber encased in steel and were designed to withstand only a few pounds of  back pressure. Compressors designed in this way relied on airend discharge check valves, oil stop valves and a fast acting sump blowdown valve to prevent the system pressure from slipping back up through the stopped rotors and pressurizing the drive side of the airend.

Lip seals made from various types of PTFE  commonly known as Teflon, came into the market in the early 80s as manufacturers discovered that the much stiffer PTFE shaft seals could withstand high pressure as well as synthetic compressor oils. They work equally well in oil free or lubricated compressors, and are easy to mass produce in those custom OEM sizes.  

Solving a shaft seal leak problem requires that you know what type of seal is used in your airend. To help in this discussion, we will divide them up according to types mentioned above. 

Mechanical seal failures 

Many mechanical seals are designed as a lubricated part. Usually there is an oil feed to and an oil scavenge from the shaft seal. The shaft seal may weep oil continuously while running if this scavenge line should become plugged. This seal scavenge line usually attaches to a fitting on the bottom of the shaft seal housing. The fitting and line should be checked before replacing the seal. In some airends, a check valve may be installed in the seal scavenge line (or in the inlet valve area where it connects) which will cause a seal to leak if it fails. 

Sometimes a leaky mechanical seal is accompanied by a jingling sound from the drive end. This might point to a broken seal spring. These coil springs are responsible for pushing the rotating seal up against the stationary seat, compensating for wear throughout it�s lifetime. However, they occasionally break, allowing the components to flop around, thus the jingling sound. Time for a new seal here, and hurry before the seal components get into the drive end bearings, creating another set of problems!  

Mechanical seals can handle more shaft runout than lip seals. However, before replacing that leaking seal,  isolate the compressor electrically, disconnect the coupling or slacken the drive belts and check the shaft side play with a dial indicator. If you have more than  .005� side play in this input shaft, the leaky seal may be pointing to an impending drive end bearing problem. 

Occasionally, the problem may not be the seal itself but instead the leak may result from a failed O-Ring. In many styles of mechanical seals, O-Rings are often used to seal the stationary seat to the seal housing, as well as the housing to the airend. A �weeping seal� problem may turn out to be a failed O-Ring or seal housing gasket. Make sure when replacing this O-Ring that the replacement is made of  the proper material to be compatible with your compressor oil and use gasket adhesive on the seal housing gasket. 

Follow the seal manufacturers instructions when replacing a mechanical seal. Be sure to lube up the inside of the rubber �boot� before assembly. Some seal manufacturers recommend using petroleum jelly, others suggest using the compressor oil or light mineral oil.  The lubricant allows the rubber sealing boot to slide on the shaft during assembly and bond to the shaft shortly afterward.  

NOTE*  When replacing the seal housing, be certain the housing and gasket line up properly with the scavenge line porting on the airend.  

Lip Seals 

Lip type shaft seals are widely used in rotary screw airends and come in a variety of designs and materials. This seal functions to retain the oil inside the airend, but also to keep air, dust and dirt from sucking in along the rotating shaft when the machine is off load. (When the inlet valve closes, if the airend continues to rotate, a vacuum may be pulled on the inlet end.)  

In general, lip seals are much more sensitive than mechanical seals to shaft runout or surface irregularities. A lip seal will often leak oil if the shaft has a total indicated runout of  more than only a few thousandths of an inch. Therefore it is even more critical before replacing a leaking lip seal to check the radial or side play in the shaft. You cannot assume because the shaft runs true when spun by hand that it will do so at speed. An out of balance coupling or drive belt pulley will cause shaft runout in an airend with a worn bearing. Tight belts will also pull the shaft away from the original centerline as bearing wear increases, putting all excess clearance on one side of the seal. Again, you should  isolate the machine, and loosen the belts or coupling. Physically push the shaft back and forth while measuring shaft deflection with a dial indicator to check for bearing wear Remember, excessive movement in this shaft, especially in belt driven airends is warning that more serious damage may be occurring inside the airend. Better to find it now.    

Rubber type shaft seals cannot withstand high pressures. A seal of this type which leaks profusely upon compressor shutdown may be an indicator of a bad airend check valve or oil stop valve, allowing full discharge pressure to reach the seal. 

When replacing a rubber lip type shaft seal, after checking to be certain that the shaft is round and running true we use a Loctite sealant between the seal and the seal housing. Press the seal into the housing and lube it up with compressor oil. We oil the shaft and, after installing new seal housing gaskets or O-Rings, slide the seal and housing onto the shaft. On shafts with no reduction to the keyed drive area, a dummy keyway plug may be required to avoid shaft damage. Take care to center the seal housing while sliding it on, ease the seal over the shaft, and carefully tighten down the housing bolts in a crossing pattern. 

PTFE shaft seals are becoming very common in newer airend designs. The use of a PTFE single or double lip shaft seal allows the compressor manufacturer to eliminate the airend check valve as well as an oil stop valve from the package. In a rare case of less is more, this not only reduces OEM cost, but eliminates without penalty, two components which have sent many airends to a premature grave. 

Like the rubber lip seals mentioned above, PTFE seals cannot tolerate shaft runout. In addition, PTFE seals have a tendency to cut a groove in the shaft due to the abrasive nature of the tough seal material and the contaminants in the oil. Therefore, most better airend manufacturers design removable shaft seal sleeves to take this wear. We highly recommend the replacement of this seal sleeve every time a PTFE seal is replaced. The seal sleeve may also have a small O-Ring seal behind it which must be replaced. If not we recommend that a Loctite sealant be used to seal the sleeve to the shaft, as hot, pressurized oil can weep under the sleeve even if the PTFE seal has seated perfectly. 

When installing a PTFE seal after replacing the seal sleeve, we apply sealant to the outside of the seal casing and press it into the seal housing. PTFE seals must be installed using a mandrel type seal installation tool. This expands the seal lips as they slide over it and  the shaft seal sleeve. DO NOT PUT OIL ON THE SEAL OR THE SHAFT SLEEVE. PTFE seals are best installed dry. When the compressor is initially started, a small amount of PTFE seal material is deposited on the sleeve, ensuring a perfect airtight seal. Be careful not to knick the inner lip of the shaft seal as any irregularity will cause a leak. If you do damage the seal attempting to put it on, sadly, you should heave it and start over with a new, if expensive, seal. Over the years, we have had very little luck repairing damaged PTFE seals, and except for an emergency, it is never wise to put an airend back together with a questionable shaft seal.


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