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Floating seal rings..learanceseals， which are free to move in a radialdirection relative to the shaft and machinehousing， are known as floating seals.These seals have advantages that veryclose， annular clearance-type seals do not possess. The floating characteristic permits them to move freelywith shaft motions and deflections， thereby avoiding the effects of severe rubbing.
.ifferential thermal expansion is a problem at high temperatures where the shaft and bushing areof dissimilar materials， or where there is anysubstantial temperature gradient between them. Forexample， the grades of carbon used commonly have a linear thermal expansion coefficient of one-third to one-fifth that ofsteel， necessitating the design of thermal expansion control into the carbon bushing. This is achieved by shrinking the carbon into a metallic retaining ring with a coefficient of expansion that equals or exceeds that of the shaft material.
It is good practice in critical applications to use bushings of a material with a slightly higher coefficient of thermal expansion than that of the shaft.Here， incipient seizure causes the bushings to grow away from the shaft. The large torque associated with high shearing intensity may necessitate locking the bushings against rotation if the unbalanced pressure forces seating them against the housing walls are insufficient to prevent rotation.
Build up of dirt or other foreign material between the seal ring and seat will result in damage to the journal and excessive seal spin on a floating sealring unit. Soft materials， such as babbitt andsilver， are notorious for trap-ping contaminants and causing shaft damage.
.echanical .Face. Seals
This device forms a running seal between flat precision-finished surfaces.Its primary function is to prevent leakage. When used on rotatingshafts， the sealing surfaces are in a plane perpendicular to theshaft， and the forces that hold the contact faces together will consequently be parallel to the shaft axis.For a seal to function properly， four sealing points must function as shown inFigure 13-25. They are: (1) the stuffing-box face must be sealed， (2) leakagedown the shaft must be sealed， (3) the mating ring in the gland plate must besealed in a floatingdesign， and (4) the dynamic faces (rotary to stationary)must seal. Basically， most mechanical seals have the following components:
1. Rotating seal ring

2. Stationary seal ring

3. Spring devices to provide pressure

4. Static seals

 

A complete seal has two basic units: the seal head unit and the seal seat.The seal head unit consists of thehousing， the end-face member， and the spring assembly. The seal seat is the mating member that completes the precision-lapped face combination that provides the seal.
The seal head may either rotate or remain stationary (attached to thebody). Either one (head or seat) may rotate， while the other remains sta-tionary. The movement of the sealing action depends on the direction of the pressure.This is illustrated in Figure13-26， which shows rotating and stationary heads.
Some form of mechanical loading device， usually a spring， is needed toensure that in the event of a loss of hydraulic pressure， the sealing surfaces are kept closed. The amount of load on the sealing area is determined by the degree of ""seal balance.'' Figure 13-27 shows what is meant by seal balance. A completely balanced combination occurs when the only force exerted onthe sealing surfaces is the springforce，i.e.， hydraulic pressure does not act on the sealing surfaces. The kind of spring that should be used depends upona variety of factors: the space available， the loading characteristics required，the environment in which the seal is to operate， etc. Based on these con-siderations， either a single spring or a multiple-spring design can be utilized.

When a very small axial space is available， bellevillesprings， finger washers， or curved washers may be used.
　
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