Mechanisms lead screw

Coarse lateral movement is required in order to locate promising regions of the surface within the dynamic range of the field of view this may be done manually, or with a stepper motor under software control, or with a mechanical lead-screw/lever arrangement, or with a so-called inchworm linear drive, or with a combination of these. 

Move the transducer with the positioning arm and aim the focal spot through a hole in the plastic plate, so that it will enter the head of the animal when it is lying on its back on the plate. The movement of the transducer arm can be accomplished with manual lead screw-based positioning system or a remote-controlled hydraulic or mechanical computer-controlled system such as described for MRI-guided focused ultrasound surgery (21) (see Note 4). 

A direct mechanical connection with the spindle drive is required to provide the proper relationship for feeding or threading operations. The lead screw is a precision part and is usually only used for threading operations to avoid unnecessary wear. 

The last quantity to be discussed in this section is maximum path of the movable mirrors. In the slow-scan instruments, usually a lead-screw is employed to drive the mirror with the synchronous motor. And a maximum scan length of the movable mirror of 5—10 cm is achieved without any problems. In the case of rapid-scan instruments however, the customer has to pay for a larger maximum scan length. From a comparison of this quantity for various instruments in Table 3 e.g. No. 4a—d and No. 5a, b) with the prices of the instruments in Table 2, we learn that with increasing prices also the maximum mirror path is increased which determines maximum resolution of the instruments as limited by the mechanics of the interferometer. We recall from our considerations in Sections 2.3, 3.2, 4.6 and 5.1 that, in Fourier spectroscopy, the resolution width is... 

There are a variety of drive mechanisms commonly used automated dispensing including cable drive, lead screw, and linear motors. Most modern equipment uses closed loop servo mechanisms, although open loop stepper motors can be found in some less expensive equipment. The drive mechanism is not as important as the overall speed and accuracy of the equipment. Other factors, such as the alignment process and dispensing technology often overshadow the characteristics of the drive mechanism. 

Considering a widely used ball screw feed drive design, the system consists of a current amplifier, servomotor, lead screw coupling mechanism, ball screw with preloaded nut, table carrying workpiece, guide friction, and feedback sensors. Figure 1 shows the overall structure. 

Control, Fig. 1 Feed drive mechanism with lead screw drive... 

Ball screw - A type of lead screw that with recirculating ball bearings between the screw and nut. This is the most driving mechanism used for linear axes of machine tools. 

Lead Screw We can convert rotary motion to linear motion by using a lead screw. This mechanism is not reversible. The metal turning lathe in Figure 10-19 is an example of a lead screw. 

The cardiovascular simulator consisted of a single channel providing mechanical excitation to the cuff of the sphygmomanometer. The cardiovascular channel was implemented as a digitally controlled linear actuator similar but not identical to that of the respiratory channels. The controller accepted angular position commands, which drove a lead screw linear actuator with a 5 mm screw lead and an available 71 mm stroke length. As with the respiratory channels, the angular position of the motor was controlled in a closed-loop manner, the loop was not closed on linear displacement. 

The Mossbauer measurement requires the generation of a precise, controllable relative motion between the source and the absorber. A large variety of drive systems has been developed. The majority of drives work on electromechanical, mechanical, hydrauHc, and piezoelectric principle. The spectrometers can be classified into constant-velodty spectrometers and velocity-sweep spectrometers. The mechanical drives, hke a lead screw or a cam, move with constant velocity. They have advantages for the thermal scan method and because their absolute velocity calibration is straightforward. The velocity-sweep spectrometers are usually of electromechanical nature (like loudspeaker-type transducers) and normally used in conjunction with a multichannel analyzer. The most commonly used M(t) functions are rectangular (constant velocity), triangular (constant acceleration), trapezoidal, and sinusoidal. A typical Mossbauer spectrometer is shown schematically in O Fig. 25.24. 

In 2007, MIT professor Hugh Herr demonstrated the first powered ankle prosthetic device [8], Mechanical compliance is one key to the proper operation of an ankle prosthetic since the foot is constantly interacting with surfaces of varying compliance. The MIT ankle does not use EPAM but rather uses a linear servomotor drive (rotary motor plus a lead screw) in combination with a spring in order to achieve a controllable compliance. A similar ankle orthotic device has also been developed [9]. Replacing this relatively heavy, bulky and noisy mechanism with an EPAM actuator would be desirable. 

FIGURE 7.41 Histograms of the distances covered (a) by the leading screw dislocations, 4, and (b) by the edge dislocations, 4, in the indentation rosettes on the surface of NaCl single crystals in dry heptane and in moist air. The indenter load was 0.3 g force. (Redrawn from Shchukin, E Physical-chemical mechanics of solid surfaces, in Encyclopedia of Surface and Colloid Science, 2nd edn., Taylor Francis, New York, 2012, pp. 1-23.)... 

The purpose of this monograph is to present a comprehensive study of the dynamics of the lead screw drives focusing on the effects of dry friction. Using a unified framework for the dynamic modeling of lead screw drives, the above three friction-instability mechanisms are studied in detail. For each instability mechanism, instability conditions are derived and the vibratory behavior of the system is studied. 

Olofsson and Ekerfors [18] investigated the friction-induced noise of screw-nut mechanisms. They discussed the tribological aspects of lubricated interaction between lead screw and nut threads, which accounts for the Stribeck friction. Based on experimental results, they have concluded that (a) the squeaking noise is the result of self-excited vibration between lead screw and nut threads (b) in the system studied (consisting of a long and slender screw), these vibrations excite... 

In a study of the effect of friction on the existence and uniqueness of the solutions of the equation of motion of dynamical systems, Dupont [19] considered a 1-DOF model of a lead screw system. He investigated the situations under which no solution existed and clearly identified one of the sources of instability in the lead screw systems i.e., the kinematic constraint instability mechanism. For the selflocking screws, he found that there is a certain limiting ratio between the lead screw moment of inertia (rotating part) and the mass of the translating part, beyond which no solution exists. 

It is worth mentioning that lead screw drives were also used in redundant positioning systems for only coarse table motion [24, 25]. In these systems, a high-precision parallel positioning system such as a piezoelectric actuator is used for fine-tuning. Another example is the work by Sato et al. [32], where they introduced an active lead screw mechanism. By using two nuts connected together by a piezoelectric actuator, they were able to actively control backlash to achieve position accuracy of better than 10 nm. 

The above force mechanisms are shared by both fastening screws and translating screws. The screws in the latter group - studied in this monograph - are commonly known as lead screws and are used for transmitting force and/or positioning by converting rotary to translational motion. In power transmission apphcations, lead screws are also known as power screws [34, 35]. When used in vertical apphcations, these systems are sometimes called screw jacks [1]. 

For design and selection purposes, the mechanical analysis of lead screws usually is limited to the factors affecting their static or quasi-static performance, such as efficiency, driving torque requirements, and load capacity [33-35]. There are... 

Wherever sliding motion exists in machines and mechanisms, friction-induced vibration may occur, and when it does, it severely affects the function of the system. Excessive noise, diminished accuracy, and reduced life are some of the adverse consequences of friction-induced vibration. To this end, lead screw systems are no exception the lead screw threads slide against meshing nut threads as the system operates. 

In Sect. 4.2, the mode coupling instability mechanism is considered. In Chap. 3, we have seen the effect of nonconservative forces in creating circulatory systems capable of exhibiting flutter instability. Examples are presented in this section to study the flutter instability with or without friction. Material presented in this section is a prelude to Chap. 7 where we study the mode coupling instability in the lead screw drives. 

Finally, in Sect. 4.3, we turn our attention to the kinematic constraint instability. This section begins by the introduction of the Painleve s paradoxes which play an important role in the kinematic constraint instability mechanism. The self-locking property which is another consequence of MctiOTi in the rigid body dynamics is also discussed in this section. This effect has a prominent presence in the study of the lead screws in Chaps. 7 and 8. The concepts presented here form the basis for the study of the kinematic constraint instability in the lead screws in Chap. 8. 

In Sects. 4.3.1 and 4.3.2, we study the classic Painleve s example and derive the conditions for the occurrence of the paradoxes. In Sect. 4.3.3, the concept of self-locking is introduced which is closely related to the kinematic constraint instability mechanism. In the rigid body systems, this phenomenon is sometimes known as jamming or wedging [97]. As we will see later on, the self-locking is an important aspect of the study of the dynamics of the lead screws. In Sect. 4.3.4, a simple model of a vibratory system is analyzed where the kinematic constraint mechanism leads to instability. In the study of disc brake systems, similar instability mechanism is sometimes referred to as sprag-slip vibration [7]. Some further references are given in Sect. 3.3.5. 

The velocity-dependent friction model used in this work is discussed in Sect. 5.1. The dynamics of a pair of meshing lead screw and nut threads is studied in Sect. 5.2. Based on the relationships derived in this section, the basic 1-DOF lead screw drive model is developed in Sect. 5.3. This model is used in Chaps. 6 and 8 to study the negative damping and kinematic constraint instability mechanisms, respectively. A model of the lead screw with antibacklash nut is presented in Sect. 5.4, and the role of preloaded nut on the increased friction is highlighted. Additional DOFs are introduced to the basic lead screw model in Sects. 5.5 to 5.8 in order to account for the flexibility of the threads, the axial flexibihty of the lead screw supports, and the rotational flexibility of the nut. These models are used in Chaps. 7 and 8 to investigate the mode coupling and the kinematic constraint instability mechanisms, respectively. Finally, in Sect. 5.9, srane remarks are made regarding the models developed in this chapter. 

Additional elements The study of lead screw drives, or any other mechanical system for that matter, can be augmented by other connected mechanical elements (e.g., a vibrating component on the moving part, additional DOF due to the flexibility of the moving part, external time-dependent forcing, etc.). These cases are outside the scope of this work and, depending on the problem they represent, may warrant a separate study. 

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