Keys，Couplings and Seals A key is the machinery component placed at the interface between a shaft and the hub of a powertransmitting element for the purpose of transmitting torque(see Figure 16(a)The key is demountable to facilitate assembly and disassembly of the shaft systemIt is installed in an axial groove machined into the shaft，called a keyseatA similar groove in the hub of the powertransmitting element is usually called a keyway，but it is more properly also a keyseatThe key is typically installed into the shaft keyseat first；then the hub keyseat is aligned with the key，and the hub is S1 d into position Figure 16 Parallel keys 1Square and Rectangular Pa rallel KeysThe most common type of key for shafts uD to 65 inches in diameter is the square key，as illustrated in Figure 16(b)The rectangular key(illustrated in Figure 16(c)is recommended for larger shafts and is used for smaller shafts where the shorter height can be toleratedBoth the square and the rectangular keys are referred to as parallel keys because the top ，bottom and the sides of the key are parallel The keyseats in the shaft and the hub are designed so that exactly onehalf of the height of the key is bearing on the side of the shaft keyseat and the other half on the side of the hub keyseat 2CouplingsThe term coupling refers to a device used to connect two shafts together at their ends for the purpose of transmitting powerThere are two general types of couplings ：rigid and flexible1)Rigid CouplingsRigid couplings are designed to draw two shafts together tightly so that no relative motion can occur between them This design is desirable for certain kinds of equipment in which precise alignment of two shafts is required and can be providedIn such cases，the coupling must be desioned to be caoable of lransmittina the tnrn=Ip in thp thoft。A typical rigid coupling is shown in Figure 1 7，in which flanges are mounted on the ends of each shaft and are drawn together by a series of bolts The load path is then from the driving shaft to its flange ，through the bolts ，into the mating flange，and out to the driven shaftThe torque places the bolts in shear The total shear force on the bolts depends on the radius of the bolt circle and the torque Rigid couplings should be used only when the alignment of the two shafts can be maintained very accurately，not only at the time of instaIIatjOn but aIsO during Operation Of the Figure 17 Rigid c。upling machinesIf significant angular，radial，or axial misal ignment occurs，stresses that are difficult to predict and that may lead to early failure due to fatigue will be induced in the shaftsThese difficulties can be overcome by the use of flexible couplings 2)Flexible CouplingsFlexible couplings are designed to transmit torque smoothly while permitting some axial， radial，and angular misalignmentThe flexibility is such that when misalignment does occur，parts of the coupling move with little or no resistance Thus，no significant axial or bending stresses are developed in the shaft Many types of flexible couplings are available commerciallyEach of them is designed to transmit a given limiting torque The manufacturerS catalog lists the design data frOm which you can choose a suitable couplingRemember that torque equals power divided by rotational speedSo for a given size of coupling，as the speed of rotation increases，the amount of power that the coupling can transmit also increases ，although not always in direct proportionOf course，centrifugal effects determine the uDDer Iimjt nf snppcourse ，centrifugal effects determine the upper limit of speed3 SeaISSeals are an important part of machine apply： design in situations where the following conditionsContaminants must be excluded from critical areas of a machineLubricants must be contained within a spacePressurized fluids must be contained within a component such as a valve or a hydraulic cylinderSome of the parameters affecting the choice of the type of seal ink system，the materials used，and the details of its design are as follows：The nature of the fluids to be contained or excluded Pressures on both sides of the seal The nature of any relative motion between the seal and the mating cOmpOnents Temperatures on all parts of the seal ing systemThe degree of sealing required Is some small amount of leakage permissible? The nature of the solid materials against which the seal must act：corrosion potential， smoothness，hardness，wear res istanceEase of service for replacement of worn seal ing elementsl he number of designs for sealing systems is virtually limitless，and only a brief Overview will be presented hereOften-designers rely on technical information provided by manufacturers of complete sealing systems or specific sealing elementsAlsoin critical Or unusual situatiOns， testing of a proposed design is advised A 键和轮毂轴之间的界面下的机械组件 电发送单元，用于传递转矩的目的（见图 1.6 （一） ） 。 关键是可拆除的，以方便组装和拆卸的轴 system.