Consider the transfer mechanisms with which you can convert rotational motion into translational or oscillatory(and vice versa).

Such mechanisms are characterized transfer function is the first derivative of move functions 1 of the driven link according to the angle of rotation or linear movement of the leading link.

Link mechanisms . An example of a lever mechanism is toggle mechanism(see fig. 1.2).

On fig. 1.11 shows a kinematic diagram of a crank-slider mechanism, which includes crank 1, connecting rod 2 and creeper 3.

This mechanism serves to convert the rotational motion of the crank 1 into the reciprocating motion of the slider 3 (and vice versa).

Rice. 1.11. Crank-slider mechanism

The transfer function is the dependence of the speed of movement of the slider on the angular velocity of the crank: v 3 =f( 1) (and vice versa).

Transmission screw-nut . On fig. 1.12 shows the screw-nut transmission, which is designed to convert the rotational movement of one link into the translational movement of another.

The transfer function is the dependence of the speed of the axial movement of the nut on the angular speed of the screw: v 2 =f( 1).

Rice. 1.12. Screw-nut transmission: 1 - screw, 2 - nut

Cam mechanism . On fig. 1.13 is given cam mechanism(which includes cam 1 and pusher 2) and its kinematic scheme.

Rice. 1.13. Cam mechanism: 1 - cam, 2 - pusher

The transfer function is the dependence of the speed of the axial movement of the pusher on the angular velocity of the cam: v 2 =f( 1).

In mechanical engineering, cam mechanisms are widespread that convert rotational motion into reciprocating or reciprocating motion: for example, to perform various operations in control systems for the working cycle of technological machines, machine tools, engines, etc. one .

Examples for Module 1 Topics

Example 1 .

The scheme of the machine is given in fig. 1.1. Motor shaft speed = 3000 rpm Angular speed of rotation of the input shaft of the actuator \u003d 2s -1. Choose a worm gear, given that the number of turns (visits) worm is equal to one or two. Define and .

Solution.

1. Let's determine the angular speed of rotation of the motor shaft (see formula (1.4)):

2. Find the gear ratio of rotation transmission (see formula (1.1)):

.

3. Let's pick up a worm gear.

Option 1. If the number of turns of the worm
, then the number of teeth of the worm wheel from the formula (1.11)

.

Option 2. If the number of turns of the worm \u003d 2, then the number of teeth of the worm wheel

Example 2

The gear train should reduce the frequency of rotation of the shaft 4 (see Fig. 1.4) by 3 times. Determine the number of teeth of the wheel if the number of gear teeth = 25.

Solution.

The number of teeth of the wheel from the formula (1.6)

.

Example 3

Rice. 1.14. For example 3

Determine the gear ratio of the mechanism shown in fig. 1.14, for a given number of gear teeth: =22, =77, =25, =50. Find the angular velocity and rotational speed of the drive shaft 1 if the shaft 3 rotates with a frequency =300 rpm.

Solution.

1. Determine the gear ratio of the gear set on shafts 1 and 2

2. Determine the gear ratio of the gear set on shafts 2 and 3

3. Gear ratio of the mechanism

4. Find the frequency of rotation of shaft 1:

5. Calculate the angular speed of rotation of shaft 1:

Answer: the gear ratio of the mechanism is 7, the speed of shaft 1 is 2100 rpm, the angular speed of rotation is 219.8 s -1.

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Mechanisms for converting rotational motion into rectilinear or reciprocating.

To convert rotational motion into rectilinear or reciprocating motion, crank, rocker, cam, hydraulic and pneumatic mechanisms are used in machine tools.

Crank mechanisms consist of a crank disk with a trunnion and a connecting rod pivotally connected to it. The course of the slider driven by the connecting rod is changed by rearranging the crank pin on the disk in the radial direction.

Rice. 125.

rocker mechanism consists of a crank disk 1, a backstage 2 (Fig. 125), swinging around axis 3. The other end of the backstage is connected to a slider 4. When the crank disk rotates, pin 5, which enters the rocker stone 6, causes the backstage to swing around axis 3.

At the same time, the rocker slides into the slots in the rocker. When changing the radius R of the crank by rearranging its pin in the radial slot of the disk, the stroke of the slider 4 changes.

Cam mechanisms divided into cylindrical and disk. The former consist of a cylinder with a copier groove or protrusion, along which, when the cylinder rotates, a finger with a roller connected to a slider slides. The second ones are profiled cams with fingers with rollers resting against their peripheral surface. These fingers are connected to sliders which are reciprocated as the cams rotate.

AT hydraulic drives the movement of the piston transmitted to the slider is carried out when oil is pumped by a gear or vane pump alternately into the cylinder cavities located on both sides of the piston.

Changing the stroke length of the slider is made by rearranging the stops acting on the lever. The latter changes the position of the spool, which closes and opens in turn the windows of the channels for inlet and outlet of oil from the cylinder.

A transmission is a technical device for transmitting one or another type of movement from one part of the mechanism to another. Transfer occurs from the source of energy to the place of its consumption or transformation. The first transmission mechanisms were developed in the ancient world and were used in the irrigation systems of Ancient Egypt, Mesopotamia and China. Medieval mechanics greatly improved motion-transmitting devices and developed many new types, using them in both spinning wheels and pottery. The true flourishing began in the New Age, with the introduction of production technologies and precise processing of steel alloys.

Various types of gears are used in various machine tools, household appliances, vehicles and other mechanisms.

The following types of transmission are usually distinguished :

• rotational movement;
• rectilinear or reciprocating;
• movement along a certain path.

Rotary gears are the most widely used type of mechanical gears.

Features of the gear mechanism

Such mechanisms are designed to transfer rotation from one gear to another using the meshing of the teeth. They have relatively low friction losses compared to friction clutches, since tight pressing of the wheelset to each other is not needed.

A pair of gears converts the speed of rotation of the shaft in inverse proportion to the ratio of the number of teeth. This ratio is called . Thus, a wheel with five teeth will rotate 4 times faster than a 20-tooth wheel in mesh with it. The torque in such a pair will also decrease by 4 times. This property is used to create gearboxes that reduce the speed of rotation with increasing torque (or vice versa).

If you need to get a large gear ratio, then one pair of gears may not be enough: the gearbox will turn out to be very large. Then several consecutive pairs of gears are used, each with a relatively small gear ratio. A typical example of this type is an automobile gearbox or a mechanical watch.

The gear mechanism is also capable of changing the direction of rotation of the drive shaft. If the axes lie in the same plane, bevel gears are used, if in different ones, then the transmission is of a worm or planetary type.

To implement the movement with a certain period, one (or several) teeth are left on one of the gears. Then the secondary shaft will move to a given angle only every complete revolution of the input shaft.

If you turn one of the gears onto a plane, you get a gear rack. Such a pair can convert rotational motion into a rectilinear one.

Gear Parameters

In order for the gears to mesh and effectively transmit motion, it is necessary that the teeth exactly match each other along the profile. The main parameters used in the calculation are regulated:

• The diameter of the starting circle.
• The engagement pitch is the distance between adjacent teeth, determined along the pitch circle line.
• Module. – The ratio of the step to the constant π. Equal modulus gears always mesh, regardless of the number of teeth. The standard prescribes a valid range of module values. Through the module, all the main parameters of the gear are expressed.
• Tooth height.

Important parameters are also the height of the head and base of the tooth, the diameter of the circumference of the protrusions, the angle of the contour, and others.

Advantages

Gear transmissions have a number of obvious advantages. It:

• transformation of motion parameters (speed and torque) over a wide range;
• high fault tolerance and service life;
• compactness;
• low losses and high efficiency;
• small axle loads;
• gear ratio stability;
• easy maintenance and repair.

Flaws

Gear mechanisms also have certain disadvantages:

• During manufacturing and assembly, high precision and special surface treatment are required.
• Unavoidable noise and vibration, especially at high rpm or high effort
• The rigidity of the structure leads to breakage when the driven shaft is locked.

When choosing the type of transmission, the designer compares the advantages and disadvantages for each specific case.

Mechanical transmissions

Mechanical transmission serves to transfer rotation from the drive shaft to the driven one, from the place of mechanical energy generation (usually an engine of one type or another) to the place of its consumption or transformation.

As a rule, motors rotate their shaft with a limited range of changes in speed and torque. Consumers require wider ranges.

According to the method of transferring mechanical energy, the following types of gears are distinguished:

• jagged;
• screw;
• flexible.
• friction.

Gear transmission mechanisms, in turn, are divided into such types as:

• cylindrical;
• conical;
• Novikov profile.

According to the ratio of the speed of rotation of the driving and driven shafts, gearboxes (reducing speed) and multipliers (increasing speed) are distinguished. A modern manual gearbox for a car combines both types, being both a gearbox and a multiplier.

Functions of mechanical gears

The main function of mechanical transmissions is to transfer kinetic energy from its source to consumers, working bodies. In addition to the main one, transmission mechanisms also perform additional functions:

• Change in speed and torque. At a constant amount of motion, the changes in these quantities are inversely proportional. Replaceable gear pairs are used for step changes, belt or torsion variators are suitable for smooth changes.
• Change of direction of rotation. Includes both a conventional reverse and a change in the direction of the axis of rotation using conical, planetary or cardan mechanisms.
• Transformation of types of movement. Rotational to rectilinear, continuous to cyclic.
• Distribution of torque between several consumers.

Mechanical transmissions perform other auxiliary functions.

Mechanical engineers have adopted several classifications depending on the classifying factor.

According to the principle of operation, the following types of mechanical transmissions are distinguished:

• engagement;
• rolling friction;
• flexible links.

In the direction of change in the number of revolutions, reducers (decrease) and multipliers (increase) are distinguished. Each of them changes the torque accordingly (in the opposite direction).

According to the number of consumers of the transmitted rotational energy, the type can be:

• single-threaded;
• multithreaded.

According to the number of conversion stages - single-stage and multi-stage.

On the basis of the transformation of types of movement, such types of mechanical transmissions are distinguished as

• Rotational translational. Worm, rack and screw.
• Rotational-rocking. Lever couples.
• Translational-rotational. Crankshafts are widely used in internal combustion engines and steam engines.

To ensure movement along complex predetermined trajectories, systems of levers, cams and valves are used.

Key indicators for choosing mechanical gears

The choice of transmission type is a complex design task. It is necessary to choose a type and design a mechanism that most fully meets the technical requirements formulated for this unit.

When choosing, the designer compares the following main factors:

• experience of previous similar designs;
• power and torque on the shaft;
• the number of revolutions at the inlet and outlet;
• required efficiency;
• weight and size characteristics;
• availability of adjustments;
• planned operational resource;
• production cost;
• service cost.

At high transmitted powers, a multi-threaded gear type is usually chosen. If you need to adjust the number of revolutions in a wide range, it would be wise to choose a V-belt variator. The final decision is up to the designer.

Cylindrical gears

Mechanisms of this type are performed with internal or external gearing. If the teeth are at an angle to the longitudinal axis, the gear is called helical. As the angle of inclination of the teeth increases, the strength of the pair increases. The helical gearing is also characterized by better wear resistance, smooth running and low noise and vibration levels.

If it is necessary to change the direction of rotation, and the shaft axes lie in the same plane, a bevel gear type is used. The most common angle of change is 90°.

This type of mechanism is more difficult to manufacture and install and, like the helical one, requires strengthening of the supporting structures.

A bevel gear can transfer up to 80% of the power compared to a cylindrical gear.

Rack and belt gear

Standards

The main parameters of various types of transmissions are normalized by the relevant GOSTs:

• Toothed cylindrical: 16531-83.
• Worm gear 2144-76.
• Involute 19274-73.

Download GOST 16531-83

In construction machines, various mechanisms are used to convert rotational motion into other types of motion in order to transfer this motion to the working body.

Rack and pinion, screw and rocker

In construction machines, to convert rotational motion into other types of motion in order to transfer this motion to the working body, various mechanisms.

rack and pinion
Design: driving gear and driven gear rack.

It is used to convert rotational motion into translational.
Design: lead screw and driven nut.

It is used to convert rotational motion into translational.
Design: driving cam and driven rod with spring.

Design: eccentric, connecting rod, slider.

It is used to convert rotational motion into reciprocating motion.
Construction: drive crankshaft with crooked spike, driven connecting rod, slider.

It is used to convert the rotational movement into the rocking movement of the wings.
Design: driving disk, slider, driven rocker.
Used in concrete pumps.

Maltese mechanism It is used to convert continuous rotating motion into intermittent rotating motion.
Design: drive disc with lever, driven maltissa.

Ratchet mechanism It is used to convert rotary motion into intermittent rotary motion, but with stop and brake.
Design: leading element - ratchet, driven - pawl (stop element).