1Therotor(rotatingmember)inanelectricmotorhasrotationalinertia,andatorqueisrequiredtobringituptospeedwhenthemotorisstarted.Ifthemotorshaftisrigidlyconnectedtoaloadwithalargerotationalinertia,andthemotorisstartedsuddenlybyclosingaswitch,themotormaynothavesufficienttorquecapacitytobringthemotorshaftuptospeedbeforethewindingsinthemotorareburnedoutbytheexcessivecurrentdemands.Aclutchbetweenthemotorandtheloadshaftswillrestrictthestartingtorqueonthemotortothatrequiredtoacceleratetherotorandpartsoftheclutchonly.电机的转子（转动元件之一）是有惯性的。当电机启动时，需要一个力矩使转子提升速度。如果电机轴与负荷是刚性连接，就会有更大的惯性。这样启动，电机在还没有足够的力矩使转子提升到一定速度，可能就被过电流烧毁了线圈。在电机和负荷之间的离合器，会限制启动转矩来保证有足够的力矩只加速转子和离合器。有些机床，可以非常方便的在电机连续运转时，通过操作离合器来启动或停止机器的。2Tokeepthepartsseparated,itisnecessarythatthepressurewithinthelubricationfilmbalancetheloadontheslidingsurfaces.Ifthelubricatingfilm’spressureissuppliedbyanexternalsource,thesystemissaidtobelubricatedhydro-statically.Ifthepressurebetweenthesurfacesisgeneratedasaresultoftheshapeandmotionofthesurfacesthemselves,however,thesystemishydro-dynamicallylubricated.Thissecondtypeoflubricationdependsupontheviscouspropertiesofthelubricant.为了把零件隔离开，润滑膜中的压力必须和作用在滑动面上的负载（荷）保持平衡。如果润滑膜的压力是由外部来源提供，该系统被认为是流体静压润滑。如果润滑表面之间的压力是由于滑动面本身的形状和运动所共同产生的，这种系统就称为流体动压润滑。第二种润滑与润滑剂的粘性密切相关。3Thematerialmustresistdeformationandfracture.Hardnessmustbebalancedagainstelasticity.Theframemustwithstandimpact,yetyieldunderloadwithoutcrackingorpermanentlydeforming.Theframematerialmusteliminateorblockvibrationtransmissiontoreduceoscillationsthatdegradeaccuracyandtoollife.Itmustwithstandthehostileshop-floorenvironment,includingthenewercoolantsandlubricants.Materialexpansionmustbeunderstood(inorder)tominimizeforcesneededtomoveslides.Thematerialmustnotbuilduptoomuchheat,mustretainitsshapeforitslifetime,andmustbedenseenoughtodistributeforcesthroughoutthemachine.材料必须能够抵抗变形和断裂。硬度必须与其弹性相平衡。机身必须能承受冲击且在载荷作用下不能裂缝和永久性变形。机身材料具有消除或阻止振动传递的性能。以减轻降低精度和刀具寿命的振荡。它必须面对不利的车间环境，包括冷却液和润滑剂。材料的膨胀不能妨碍滑动部件的运动。在正常工作期间材料不能产生太多的热量，应能保护其形状，且具有足够大的密度以支承机床的其它