Glossary
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AAbsolute MoveA positioning movement where the distance is referenced to a fixed original position. For example, if a stage is positioned at +500 mm, an absolute move to +300 mm would result in a move of 200 mm towards the origin (in the negative direction). AccelerationThe change in velocity as a function of time, going from a lower speed to a higher speed. AccuracyAn absolute measurement defining the difference between expected and actual position. Acme ScrewA leadscrew which uses an ACME threaded screw design with sliding surfaces between the screw and nut.
The Acme thread form uses a 29 degree thread angle with flat apex and valley. The Acme screw thread is stronger than V-profile 60 degree threads. Wear can be compensated for with a split nut. Typically found where large loads or accuracy is required as in vises or the lead screw of a lathe. Standardized variations include multiple-start threads, left-hand threads, and self-centering threads which are less likely to bind under lateral forces. ActuatorA self contained leadscrew system which converts rotary motion (from a motor) to linear motion. AllowanceThe prescribed difference between the design (maximum material) size and the basic size. BBackdriveTendency of a cylinder to creep out of its set position due to an applied load or force. BacklashThe amount of play between a leadscrew and nut. Backlash causes lost motion when changing the direction of travel. Backlash is determined by measuring the range of angular movement of the driven shaft which results in no motion of the positioning table. Backlash is typically seen in drive trains, leadscrews and bearings. Ball ScrewA leadscrew which uses a ball nut which houses one or more circuits of recirculating steel balls which roll between the nut and screw.
Basic SizeThe nominal size of the screw thread being produced. The tolerance is applied to the basic size to determine the maximum and minimum acceptable dimension. BearingA support device which allows a smooth, low friction motion between two surfaces loaded against each other. BushingA cylindrical metal or polymer sleeve inserted into a machine part to reduce friction between moving parts. CClosed Loop PositioningThe use of feedback devices (encoders, resolvers, interferometers, etc.) to allow a motor to position a load accurately. Coefficient of FrictionThis is defined as the ratio of the force required to move a given load to the magnitude of that load. Typical values for ball and crossed roller slides are 0.001 to 0.005. Constant Pitch SeriesThe UN series provides a comprehensive range of diameter pitch combinations where the Coarse, Fine, and Extra-Fine series do not satisfy the requirement of the design. They are available with 4, 6, 8, 12, 16, 20, 28 and 32 threads per inch. More details regarding the 8, 12, and 16 thread series are provided below:
DDecelerationThe change in velocity as a function of time, going from a higher speed to a lower speed. Drive RatioThe ratio of motor revolutions per leadscrew revolution. Drive TrainThe arrangement by which the motor is coupled to the leadscrew. Typically provided by gears, timing belt/pulley or direct coupling. Duty CycleThe ratio of Motor on time and total cycle time within a given cycle of operation. Duty Cycle (%) = (Motor ON Time / Total Cycle Time) X 100% Dwell TimeTime within a move cycle where no motion occurs EEfficiencyRatio of output power vs. input power. EncoderAn electromechanical device which produces discrete electrical pulses directly related to the angular position of the input shaft, providing high resolution feedback data on position, velocity, and direction. FFast LeadOur "Fast Lead" products indicate that the Thread lead is greater than or equal to 0.500" per revolution. This is the lead point at which a leadscrew will start to backdrive. The larger the thread lead past .500, the more it will tend to backdrive. Aleadscrew ( thread and nut combination) will tend to backdrive if the applied load is mounted in a position other than horizontal and the total lead is greater than .500 inches per rev. ForceThe action of one body on another which tends to change the state of motion of that body. Typically described in terms of magnitude, direction, and point of application. FrictionThe resistance to motion of two surfaces that touch. IIncremental MoveA move referenced from the last set position. InertiaProperty of an object that resists a change in motion. It is dependent on the mass and shape of the object. The greater the mass of an object, the greater its inertia, and the more force is necessary to accelerate and decelerate. LLeadThe linear distance a nut on a leadscrew will travel with one revolution of the leadscrew. Lead AccuracyThe variation of distance a nut travels in a complete turn, as compared to the nominal or theoretical distance. Another name for the amount of lead error. Lead ErrorThe deviation of a leadscrew from its nominal pitch. The error is often linear, although periodic error can be present. Also referred to as lead accuracy. Lead ScrewAnother name for Leadscrew. Pronounced LEED - screw. LeadscrewA device which converts rotary motion to linear motion. The screw and nut pair can be used to convert torque into linear force. As the screw (or threaded rod) is rotated, the non-rotating nut moves along the lead-screw, or the screw moves along its axis through the fixed nut. Left Hand ThreadA screw thread that is screwed in (tightened) by rotating counterclockwise. NOTE: This is contrary to the conventional screw thread where the tightening direction is clockwise. MMassThe quantity of matter that an object contains. MotorA device which converts electrical energy into mechanical energy. NNut RigidityThe stiffness of a leadscrew and nut assembly, typically measured in Newtons per meter. This stiffness, together with the moving mass and duplex bearing stiffness, sets the primary natural frequency of a leadscrew driven stage or slide. OOpen Loop PositioningA positioning technique, typically utilizing stepping motors, in which the controller issues a sequence of commands to the motor without any absolute means of detecting if the move has in fact been made. When the load and move velocity and acceleration are appropriately defined, open loop positioning is capable of extended operation without losing steps. PPitchThe number of revolutions a leadscrew (specified in British or American units) must turn to advance the nut one inch (single start only). This is known as TPI (Threads Per Inch or Turns Per Inch.) A thread or turn being defined as one complete 360 degree revolved profile of the thread form. For example, a 5 pitch (British) leadscrew has a lead of 0.200 inches. This is calculated by inverting the TPI or pitch, for example, Lead = 1/TPI = 1/5 = 0.200 inches However, in Metric units, pitch has also become known as the distance the nut will travel per revolution of the leadscrew. Metric screws are actually specified by lead only, in millimeters (per revolution) even if it has multiple starts. PowerHow much work is done in a specific amount of time. RRepeatabilityThe ability of a positioning system to return to an exact location during operation (from the same direction with the same load and speed). ResolutionThe smallest positioning increment achievable. In digitally programmed systems it is the smallest specifiable positioning increment. Right Hand RuleIn mathematics and physics, the right-hand rule is a convention for determining relative directions of certain vectors.
Right Hand ThreadA screw thread that is screwed in (tightened) by rotating clockwise. Most threads are right handed and follow the Right Hand Rule. So, from the end view of the thread, Rotate it to the LEFT (counterclockwise) to loosen (or move the screw toward you.) Rotate it to the RIGHT (clockwise) to tighten (or move the screw away from you.) SScrew ThreadA screw thread is a helical or tapered structure used to convert between rotational and linear movement or force. A screw thread may be thought of as an inclined plane wrapped around a cylinder or cone. Screw threads have several applications:
In all of these applications, the screw thread has two main functions:
In most applications, the thread pitch of a screw is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied so long as no external rotational force is present. This characteristic is essential to the vast majority of its uses. When the pitch is large enough to allow creep, external linear force converts to rotary motion (see backdrive.) Servo MotorA motor which is used in closed loop systems where feedback is used to control motor velocity, position, or torque. Stepper MotorMotor which translates electrical pulses into precise mechanical movements. Through appropriate drive circuitry, controlling the rate and quantity of pulses will control the motors velocity and position. Symmetrical ThreadBoth flanks of the thread profile are inclined at the same angle. TThread AngleThe angle formed by the two sides, or flanks of the thread (or their projections) with each other.
Thread AxisAn imaginary line running lengthwise through the ideal center of the screw.
Thread ClassA system used to specify the amounts of tolerance and allowance. Classes 1A, 2A and 3A apply to external threads; classes 1B, 2B and 3B apply to internal threads. Thread CrestThe top surface joining the two sides, or flanks of a thread. Thread DepthThe distance between the crest and the root of a thread.
Thread FlankThe thread flanks, or sides join the thread roots to the crest. Thread HeightThe radial distance between the minor and major diameters of the thread. Thread LengthThe length of the portion of the rod with threads. Thread MicrometerA micrometer in which the spindle is ground to a point having a conical angle of 60 degrees. The anvil, instead of being flat. has a 60 degree V-Shaped groove which fits the thread. Thread PitchThe distance from a point on one screw thread to a corresponding point on the next thread.
Thread Pitch DiameterThe diameter of a screw thread measured from the thread pitch line on one side to the thread pitch line on the opposite side. Thread RootThe bottom surface joining the sides of two adjacent threads. On external threads the roots are usually rounded so that fatigue performance is improved. Thread RunoutThe portion at the end of a threaded rod which is not cut or rolled to full depth, but which provides a transition between full depth threads and the rod shank or head. ThrustThe measurement of linear force. TorqueA measure of angular force which produces rotational motion. UUnified Screw ThreadsThese are groups of diameter-pitch combinations differentiated from each other by the numbers of threads per inch applied to a specific diameter. The various diameter-pitch combinations of eleven standard series are considered common.
VV ThreadA thread form which uses a 60 degree V threaded screw design with sliding surfaces between the screw and nut. VelocityThe change in position as a function of time. WWeightForce of gravity acting on a body. Determined by multiplying the mass of the object by the acceleration due to gravity. |
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