Laboratory plant for specimen stretching and tensile force measurement

Contents

CONTENTS

INTRODUCTION

1 FUNCTIONAL AND PHYSICAL MODELS

2 MATHEMATICAL MODEL

2.2 MATHEMATICAL MODEL OF NUT

2.3 MATHEMATICAL MODEL OF WORM GEAR

2.4 MATHEMATICAL MODEL OF SPRING

2.5 MATHEMATICAL MODEL OF LEVER

2.5.1 OPERATION WITH MATHEMATICAL MODEL OF LEVER

2.6 CAPTURE CALCULATION

2.6 BEARING CALCULATION

CONCLUSION

Task

1. Project Theme: Design of Experimental Laboratory Plant for Sample Stretching and Tensile Force Measurement

2. Initial data for work: Option according to the student's code: 150244

3. Contents of explanatory note: Introduction, calculation part, conclusion.

4. List of graphic material: Figures, tables.

Functional and Physical Models

Describe the functional and physical models of the installation elements.

1) Lever. Lever serves for creation of force on tie-rod connecting grips of installation and hinge of short arm of lever mechanism by means of increase of force acting on hinge of long arm of lever from side of screw. It is represented by a rigid beam of rectangular cross section. The criterion of lever operability is the ability to transfer force from the screw to the tested sample, without experiencing bending.

2) Screw gear. The screw gear consists of a screw and a nut, designed to convert the moment supplied to the nut into a force on the screw by interacting the parts along the screw surface. This force lifts the screw vertically up, the screw receives the transmitted energy of mechanical movement from the nut.

3) Screw. Screw is designed for creation of long-term axial compressive force by means of conversion of rotary motion into translational motion and driven moment due to interaction with nut on screw surface. It is represented as a complex part of a cylindrical shape with a helical surface. In the screw-nut transmission, the screw must prevent:

wear of the screw;

destruction of the screw by dangerous sections or threads;

absence of the required self-braking of the screw (can occur at a large angle of the thread profile);

loss of stability at maximum extended position.

4) Nut. Nut is used to perform function of rotation transfer to screw. Perceives the moment of friction forces acting on the outer cylindrical surface of the nut and the reaction of the support transmitted through the thrust bearing and the housing of the worm transmission unit to the sensitive element of the installation (compression spring). It is represented by a bushing with internal thread on a through hole, an external cylindrical surface. The following shall be prevented in the screw-nut transmission:

destruction of nut by dangerous sections of nut housing, thread turns;

sliding the nut inside the housing, which is not critical, but leads to inconvenience in the operation of the mechanism, and can also reduce the resistance of the nut to destruction.

5) Spring. The spring serves as a sensitive element that receives the force from the compression of the screw. The calculation of springs is reduced to the calculation of tangent stresses and comparison of these stresses with permissible ones.

6) Worm mechanism. The worm mechanism serves to substantially increase the torque. It consists of a worm and a worm wheel interacting on the surfaces of the teeth of the worm wheel and the screw line of the worm. The leading link is the worm. The calculation of worm gears is reduced to the calculation of contact stresses and bending stresses of the teeth of the worm wheel.

7) Worm wheel. The worm wheel transmits the moment created on it to its connection with the nut. The nut is connected to the wheel by interference fit.

8) Worm shaft assembly. The worm shaft is designed to transmit torque from the handle to the worm. All other components of the power action on the worm shaft from the side of the worm wheel are balanced by the reactions of the shaft supports, the function of which is performed by radial thrust bearings of the worm shaft.

9) The nut is connected to the wheel by interference fit.

10) Counting device. The reference device is a set of parts intended for visual determination of the measured value by observing the position of the pointer relative to the system of strokes and digits located on the scale. The main parts of the reference devices are scales and indicators.

11) Scale. Scale - a set of marks (strokes, numbers, letters) located along a straight line or along an arc of a circle and depicting a series of consecutive numbers corresponding to the values ​ ​ of the measured value. The part on which the scale is applied is called a dial .

12) Pointer. A pointer is an arrow that occupies a certain position relative to the scale and thereby marks the numerical value of the measured value.

13) Handle. Handle is intended for transformation of force of researcher's hands into torque acting on screw, which overcomes moments of forces of resistance to rotation. It is represented by a cylindrical lever.

Based on the formulated definitions, we proceed to the mathematical model of the details, formulating the criteria for their operability and expressing them through certain quantitative relations. The definition of unknown (as a rule, geometric) parameters of these models is the essence of calculating the parts of the installation.

Mathematical model

Source Data:

maximum force F = 230 kN.

2.1 Mathematical model of screw

The main criterion of screw gear operability is thread wear resistance. The intensity of wear depends on: load per unit area, material, temperature, presence of lubricant, surface roughness, sliding speed.

In order for the part to be wear resistant, it is necessary that the wear intensity is not very high, which guarantees its long-term operation.

Therefore, in a simplified form, the wear resistance condition can be written in the form:

p≤p

Conclusion

In this course design, an experimental plant was designed to stretch the sample and measure the tensile force. At the same time, calculations were made of the screw, nut, spring, worm gear, lever, grips, and bearings were selected.