英语翻译
Centrifugal force principles are also used in common appliances such as clothes dryers and in devices to control motor speeds and accelerate moving machines.Centripetal force causes an object to travel in circular path.This action is caused by the continuous application of forces,which tend to pull the object to the center.In other words,the inward force,which resists the centrifugal force,is called the centripetal force.The centripetal force of objects spinning at a constant rate produces an acceleration toward the center which is equal and opposite to the centrifugal force.The materials used in the construction of rapidly moving machine parts and mechanisms must be structurally strong enough to provide the centripetal force required to hold the parts to a circular path.At the same time,the materials must be able to withstand the centrifugal force,which tends to pull the parts apart.Motion and the basic laws,which affect motion,are important considerations because of the numerous applications of these principles to produce work through mechanical devices.There are two primary mechanical motions:rotary and rectilinear.These terms suggest that rotary motion is a circular movement around a center line and rectilinear motion is a straight line motion.For either rotary or rectilinear motion,it is possible,with added mechanical devices,to produce other forms of motion such as intermittent motion and reciprocating motion.Rotary Motion:The motion that is commonly transmitted is rotary motion.This type of motion may be produced with hand tools or power tools.Rotary motion is required to drill holes,turn parts in a lathe,mill surfaces,or drive a generator or fan belt.Rectilinear Motion:The feed of a tool on a lathe,the cutting of steel on a power saw,or the shaping of materials are all situations in which rectilinear or straight line motion produces work.In each of these situations a part or mechanism is used to change rotary motion to straight-line motion.The screw of a micrometer and the threads in a nut are still other applications where the direction of motion is changes from rotary to rectilinear.Harmonic and Intermittent Motion:Any simple vibration,such as the regular back,and forth movement of the end of a pendulum,is simple harmonic motion.However,many manufacturing processes require intermittent or irregular motion.For example,the fast return stroke of a power hacksaw or shaper ram is desirable because no cutting is done on the return stroke.Therefore,as more time is saved in returning the cutting tool to the working position,the less expensive is the operation.The combinations of rotary and rectilinear motion obtainable are unlimited because of the large variety of parts such as gears,cams,pulleys,screws,links,and belts,which can be combined in many arrangements.