Read and translate the following text.

Date: 2015-10-07; view: 403.

Wear

Text B

In material and components science, wear is the erosion of material from a solid surface by the action of another surface. It is related to surface interactions and more specifically the removal of material from a surface as a result of mechanical action. Wear can also be defined as a process in which interaction of the surfaces of a solid with its working environment results in dimensional loss of the solid, with or without loss of material. Aspects of the working environment which affect wear include loads (such as sliding, reciprocating, rolling, and impact loads), speed, temperature, type of counterbody (solid, liquid or gas), and type of contact.

The working life of an engineering component is over when dimensional losses exceed the specified tolerance limits. Wear, along with other aging processes such as fatigue, creep and fracture toughness, causes progressive degradation of materials with time, leading to failure of material.

The complex nature of wear has delayed its investigations. Up to now there has been no specific standard for testing or measuring a materials wear resistance although a number of wear tests have been developed in an attempt to standardize wear testing for specific applications.

There are at least five types of wear:

Adhesive wear is the most common form of wear and occurs when two bodies slides over each other, or are pressed into one another, which promote material transfer between the two surfaces. Adhesive wear can be described as plastic deformation of very small fragments within the surface layer when two surfaces slides against each other.

Abrasive wear occurs when a hard rough surface slides across a softer surface. Abrasive wear is commonly classified according to the type of contact and the contact environment. The type of contact determines the mode of abrasive wear. The two of abrasive wear are known as two-body and three-body abrasive wear.

Surface fatigue is a process by which the surface of a material is weakened by cyclic loading. Fatigue wear is produced when the wear particles are detached by cyclic crack growth of microcarcks on the suface. These microcaraks are either superficial cracks or subsurface cracks.

Fretting wear is the repeated cyclical rubbing between two surfaces over a period of time which will remove material from one or both surfaces in contact. It occurs typically in bearings, although most bearings have their surfaces hardened to resist the problem.

Erosive wear is caused by the impact of particles of solid or liquid against the surface of an object. The impacting particles gradually remove material from the surface through repeated deformations and cutting actions

Задания к тексту B:

I. Найдите в тексте английские эквиваленты следующим русским выражениям:

материаловедение; сложная природа износа; поверхностная усталость; срок службы технического компонента; поверхностное взаимодействие; абразивный износ; процесс старения; адгезионный износ; потеря размеров твердого тела; эрозивный износ; усталость, ползучесть и трещиностойкость; фреттинг-износ; до сих пор;

II. Сформулируйте в нескольких словах содержание каждого абзаца.

III. Напишите резюме к тексту.

IV. Ответьте на вопросы:

1. What is wear?

2. What is wear related to?

3. How can wear also be defined?

4. What do aspects of the working environment which affect wear include?

5. When is the working life of an engineering component over?

6. What causes progressive degradation of materials with time?

7. What has delayed the wear investigations?

8. There has been no specific standard for testing or measuring a materials wear resistance up to now, has there?

9. Have a number of wear tests been developed in an attempt to standardize wear testing or to minimize wear for specific applications?

10. How many types of wear do you know?

11. What are they?

V. Перескажите текст, используя в качестве плана вопросы предыдущего упражнения.

A modern motion control system typically consists of a motion controller, a motor drive or amplifier, an electric motor, and feed­back sensors. The system might also contain other components such as one or more belt-, ballscrew-, or leadscrew-driven linear guides or axis stages. All of the components of a motion control system must work together seamlessly to perform their assigned functions. Motion control systems can be found in such diverse applications as materials handling equipment, machine tool cen­ters, chemical and pharmaceutical process lines, inspection sta­tions, robots, and injection moulding machines.

Electric-powered motion control systems do not require pumps or air compressors, and they do not have hoses or piping that can leak hydraulic fluids or air. This discussion of motion control is limited to electric-powered systems.

Motion control systems can be classified as open-loop or closed-loop. An open-loop system does not require that measurements of any output variables be made to produce error-correcting sig­nals; by contrast, a closed-loop system requires one or more feedback sensors that measure and respond to errors in output variables.

A closed-loop motion control system has one or more feedback loops that continuously com­pare the system's response with input commands or settings to correct errors in motor and/or load speed, load position, or motor torque. Feedback sensors provide the electronic signals for cor­recting deviations from the desired input commands. Closed- loop systems are also called servosystems. Each motor in a servosystem requires its own feedback sen­sors, typically encoders, resolvers, or tachometers that close loops around the motor and load. Variations in velocity, position, and torque are typically caused by variations in load conditions, but changes in ambient temperature and humidity can also affect load conditions.

A velocity control loop, as shown in block diagram Fig. 4, typi­cally contains a tachometer that is able to detect changes in motor speed. This sensor produces error signals that are proportional to the positive or negative deviations of motor speed from its preset value. These signals are sent to the motion controller so that it can compute a corrective signal for the amplifier to keep motor speed within those preset limits despite load changes.

A position-control loop typically contains either an encoder or resolver capable of direct or indirect measurements of load position. These sensors gener­ate error signals that are sent to the motion controller, which pro­duces a corrective signal for amplifier. The output of the ampli­fier causes the motor to speed up or slow down to correct the position of the load. Most position control closed-loop systems also include a velocity-control loop.

The ballscrew slide mechanism, shown in Fig. 6, is an example of a mechanical system that carries a load whose position must be controlled in a closed-loop servosystem because it is not equipped with position sensors. Three examples of feedback sensors mounted on the ballscrew mechanism that can provide position feedback are shown in Fig. 7: (a) is a rotary optical encoder mounted on the motor housing with its shaft coupled to the motor shaft; (b) is an optical linear encoder with its graduated scale mounted on the base of the mechanism; and (c) is the less com­monly used but more accurate and expensive laser interferometer.

A torque-control loop contains electronic circuitry that meas­ures the input current applied to the motor and compares it with a value proportional to the torque required to perform the desired task. An error signal from the circuit is sent to the motion con­troller, which computes a corrective signal for the motor ampli­fier to keep motor current, and hence torque, constant. Torque- control loops are widely used in machine tools where the load can change due to variations in the density of the material being machined or the sharpness of the cutting tools.

If a motion control system is to achieve smooth, high-speed motion without overstressing the servomotor, the motion con­troller must command the motor amplifier to ramp up motor velocity gradually until it reaches the desired speed and then ramp it down gradually until it stops after the task is complete. This keeps motor acceleration and deceleration within limits.

The trapezoidal profile, shown in Fig. 8, is widely used because it accelerates motor velocity along a positive linear "up- ramp" until the desired constant velocity is reached. When the motor is shut down from the constant velocity setting, the profile decelerates velocity along a negative "down ramp" until the motor stops. Amplifier current and output voltage reach maxi­mum values during acceleration, then step down to lower values during constant velocity and switch to negative values during deceleration.

1. Translate the following extract into Ukrainian:

The simplest form of feedback is proportional control, but there are also derivative and integral control techniques, which com­pensate for certain steady-state errors that cannot be eliminated from proportional control. All three of these techniques can be combined to form proportional-integral-derivative (PID) control.

In proportional control the signal that drives the motor or actuator is directly proportional to the linear difference between the input command for the desired output and the measured actual output.

In integral control the signal driving the motor equals the time integral of the difference between the input command and the measured actual output.

In derivative control the signal that drives the motor is pro­portional to the time derivative of the difference between the input command and the measured actual output.

In proportional-integral-derivative (PID) control the signal that drives the motor equals the weighted sum of the differ­ence, the time integral of the difference, and the time deriva­tive of the difference between the input command and the measured actual output.