Component: independent movement unit
Mechanism: a system of components used to transmit motion and force, which has a frame and consists of components that can move relatively.
Machine: It is a device that performs mechanical movement and is used to convert or transmit energy, matter and information.
Machinery: the general term for machines and institutions.
Schematic diagram of mechanism movement: simple lines and symbols are used to represent components and motion pairs, and the relative positions of each motion pair are determined according to a certain proportion. This simple graph representing the relative motion relationship between components in the mechanism is called the mechanism motion diagram.
Kinematic pair: a movable connection consisting of two members in direct contact.
Kinematic pair element: the surface of a kinematic pair formed by contacting two components.
The relationship between the degree of freedom and the number of constraints of motion pair f=6-s
Kinematic chain: a relative motion system composed of components connected by kinematic pairs.
High pair: a kinematic pair consisting of two members in contact with points and lines.
Low pair: A kinematic pair formed by surface contact between two components.
The maximum constraint number of planar kinematic pairs is 2 and the minimum constraint number is1; A kinematic pair with one constraint is a high pair, and a kinematic pair with two constraints is a plane low pair.
The calculation formula of plane degree of freedom: f = 3n-2pl-ph.
Movable condition of mechanism: the degree of freedom of mechanism is greater than zero.
The mechanism has the conditions that determine the motion: the number of original moving parts of the mechanism should be equal to the number of degrees of freedom of the mechanism.
Virtual constraint: the constraint that does not limit the mechanism.
Local degree of freedom: the degree of freedom independent of the motion of the output mechanism.
Composite hinge: two or more components are connected in one place at the same time with a rotating pair.
Instantaneous center of velocity: the coincidence point where the instantaneous velocity is equal on two components that interact and move relative to each other. If the absolute velocity is zero, the instantaneous center is called absolute instantaneous center.
The same point between the instantaneous center of relative velocity and the instantaneous center of absolute velocity: the point where the instantaneous relative velocity is zero on the two components interacting in plane relative motion; Difference: the absolute speed of the latter is zero, while the former is not.
Three-center theorem: the three instantaneous centers of three components moving in a plane must be on the same straight line.
Instantaneous center number of mechanism: N=K(K- 1)/2.
Mechanical self-locking: in some machines, some machines can move according to their structural conditions, but due to the existence of friction, there will be cases where no matter how much driving force is added, they cannot move.
Crank: a member that rotates around a fixed shaft;
Connecting rod: a member that moves in a plane;
Rocker: a member that swings on a fixed shaft;
Connecting rod: a component connected with the frame;
Revolving pair: a kinematic pair that can rotate 360 relative to each other.
Swing pair: A pair of motion that can only swing at a limited angle.
Conditions of hinged four-bar mechanism with crank;
1. The sum of the lengths of the longest bar and the shortest bar should be less than or equal to the sum of the lengths of the other two bars, which is called the bar length condition.
2. One of the connecting rods or the frame is the shortest rod.
When the bar length condition is satisfied, the rotating pairs composed of the shortest bars are all integer rotating pairs.
Three basic forms of hinged four-bar mechanism;
1. crank rocker mechanism
Take the adjacent side of the shortest pole as the frame.
2. Double crank mechanism
Take the shortest pole as the frame.
3. Double rocker mechanism
Take the opposite side of the shortest pole as the frame.
In the crank-rocker mechanism, the length of the rocker becomes infinite, forming a crank-slider mechanism.
The eccentric wheel mechanism is formed by changing the radius of the rotating pair in the crank-slider mechanism.
Quick return movement: when the average speed of the idle return stroke of the prime mover (such as the crank of the crank-rocker mechanism) and other followers (rockers) of the planar linkage mechanism is greater than the average speed of their working stroke.
Pole angle: when the mechanism is at two poles, the angle θ between the two positions where the prime mover AB is located.
θ= 180(K- 1)/(K+ 1)
Stroke speed ratio coefficient: the ratio of the average speed V2 of the follower's no-load return trip to the average speed V 1 is used.
k = V2/v 1 =( 180+θ)/( 180-θ)
Whether the planar four-bar mechanism has quick return characteristics depends on the size of the limit included angle.
The bigger θ is, the bigger K is, and the more obvious the nature of snap-back motion is. When θ=0 and K= 1, there is no quick return characteristic.
Four-bar mechanism with quick return characteristics: crank-slider mechanism, offset crank-slider mechanism and swinging guide rod mechanism.
Pressure angle: the angle (acute angle) α between force F and the positive direction of velocity V at point C.
Transmission angle: the angle complementary to the pressure angle (acute angle) γ.
In the crank-rocker mechanism, only when the rocker is a moving part can the dead center position appear. When in the dead center position, the transmission angle γ of the mechanism is 0.
Although the dead center position is not good for transmission, in engineering practice, sometimes the dead center position of the mechanism can be used to complete some work requirements.
Rigid impact: infinite acceleration and inertia force will cause great impact on the cam mechanism (for example, the follower moves at a uniform speed).
Flexible impact: the acceleration suddenly becomes limited, thus causing a small impact (for example, the follower is simple harmonic motion).
Among several basic follower motion laws of cam mechanism, constant velocity motion law makes cam mechanism produce rigid impact, constant acceleration and equal deceleration, cosine acceleration motion law produces flexible impact, and sine acceleration motion law has no impact.
Among all kinds of common push rod motion laws of cam mechanism, constant velocity is only suitable for low speed; Equal acceleration, equal deceleration and cosine acceleration are suitable for medium speed, while sine acceleration can move at high speed.
Cam base circle: The circle drawn with the minimum radial diameter r0 of the cam profile as the radius is called the base circle.
The radius of the cam base circle is the shortest distance from the rotation center to the cam profile. The smaller the radius of the cam base circle, the greater the pressure angle of the cam mechanism and the smaller the size of the cam mechanism.
The pressure angle α of the cam mechanism is the acute angle between the moving direction V of the follower and the force F.
Offset e: the distance from the cam rotation center of the follower guide rail.
Offset circle: a circle drawn with e as the radius and cam rotation center as the center.
Push stroke: the process that the follower is pushed by the cam profile and reaches the farthest position from the rotation center according to a certain motion law.
Lifting height h: the distance that the push rod moves.
Return stroke: the process that the follower returns to the initial position from the farthest position from the rotation center under the action of spring or gravity with a certain motion law.
Motion angle: the angle of rotation when the cam moves.
The basic law of tooth profile meshing: the transmission ratio of a pair of gears meshing with each other at any position is inversely proportional to the common normal of the meshing tooth profile divided by the length of two line segments at the contact point.
Involute: the trajectory AK of any point K on the straight line when the straight line BK makes pure rolling along the circumference.
Characteristics of involute:
The length of BK line segment on 1. bus is equal to the arc length AB rolling on the base circle.
2. The hairline of any point on an involute is tangent to its base circle.
3. The closer the involute is to the base circle, the smaller the radius of curvature is, and its radius of curvature is zero on the base circle.
4. The shape of involute depends on the size of base circle.
5. There is no involute in the base circle.
6. Any two common normals corresponding to any arc length on the same base circle are equal.
Meshing characteristics of involute tooth profile;
1, which can ensure a fixed transmission ratio and is separable.
The transmission ratio is not only inversely proportional to the pitch circle radius, but also inversely proportional to the base circle radius and indexing circle radius.
I 12 =ω 1/ω2 = O2P/o 1P = rb2/Rb 1
2. The positive pressure direction between involute tooth profiles remains unchanged.
Basic parameters of involute gear: modulus, number of teeth, pressure angle, (tooth height coefficient, backlash coefficient)
Modulus: artificially specified: m=p/π can only take some simple values.
Diameter of cyclotron: d=mz, r = mz/2.
Height of tooth tip: ha=ha*m
Height of tooth root: HF = (ha *+c *) m.
Diameter of addendum circle: da = d+2ha = (z+2ha *) m.
Diameter of root circle: df = d-2hf = (z-2ha *-2c *) m.
Diameter of base circle: db= dcosα= mzcosα.
Tooth thickness and tooth slot width:? s=πm/2 e=πm/2
Standard center distance: a = r1+R2 = m (z1+z2)/2.
The condition for a pair of involute gears to mesh correctly is that the modulus and pressure angle of the two wheels are equal respectively.
When a pair of involute tooth profiles mesh, the contact point is on the actual meshing line, and the theoretical meshing line length is the internal common tangent N 1N2 of the two base circles.
The pressure angle of any point on the involute tooth profile refers to the included angle between the normal direction and the speed direction of that point.
The normal of any point on the involute tooth profile is tangent to the base circle.
According to its principle, tooth cutting methods can be divided into forming method (copying method) and generating method.
Undercutting: The reason why undercutting occurs when involute tooth profile is cut by generating method is that the top line of cutter teeth exceeds the meshing limit point N 1 (the minimum number of teeth without undercutting for standard gear is 17 and for helical gear is 14).
Coincidence: the ratio ε of b 1 B2 to Pb;
Continuous condition of gear transmission: coincidence ε is greater than or equal to 1.
Improved gear:
Based on the position when cutting the standard gear, the distance xm moved by the cutter is called displacement, and X is called displacement coefficient. It is stipulated that when the cutter is far away from the center of the wheel blank, X is positive, which is called positive displacement. When the tool approaches the wheel blank, X is negative, which is called negative displacement.
The pitch, modulus, pressure angle, base circle and indexing circle of the modified gear remain unchanged, but the tooth thickness and groove width on the indexing line are not equal.
Tooth thickness: s=πm/2+ 2xmtgα
Cogging width: e = π m/2-2xmtgα.
Spiral gear:
Conditions for correct meshing of a pair of helical cylindrical gears;
mn 1=mn2，αn 1？ =αn 1 external engagement:? β 1=-β2
Or mt 1=mt2, α t 1 = α t2 external engagement:? β 1=-β2
The parameters of the normal plane take standard values, while the geometric dimension calculation is carried out on the end face.
Modulus: mn=mtcosβ
Diameter of cyclotron:? d=zmt=z mn / cosβ
Definition of helical cylindrical gear equivalent gear: An imaginary straight gear with normal tooth profile equivalent to helical cylindrical gear is called helical cylindrical gear equivalent gear.
Equivalent number of teeth: Zv=Z/cos3β.
Gear train: A transmission system consisting of a series of gears.
Fixed axis gear train: if the axis of each gear tooth is fixed relative to the frame when the gear train is running.
Epicyclic gear train: If the position of at least one gear shaft is not fixed, but it rotates around the fixed shaft of other gears during continuous operation.
Compound gear train: fixed axis gear train+epicyclic gear train
The epicyclic gear train with 1 degree of freedom is called planetary gear train, and the epicyclic gear train with 2 degrees of freedom is called differential gear train.
The transmission ratio of the fixed-axis gear train is equal to the ratio of the continuous product of all driven gear teeth to the continuous product of all driving gear teeth.
I1m = (-1) m = (-1) m the product of the number of teeth of all driven wheels/the product of the number of teeth of all driving wheels.
Transmission ratio of planetary gear train;
or
Intermediate wheel: it does not affect the transmission ratio, but only plays the role of intermediate transition and changing the steering of driven wheels.
Calculation of transmission ratio of compound gear train;
1. Differentiate the gear train: First, find the planetary gear whose axis is not fixed, whose axis is the planetary carrier, and the gear directly engaged with the gear with fixed axis is the sun gear, which is a basic epicyclic gear train. After all epicyclic gear trains are separated, what remains is the fixed axis gear train.
2. The formulas for calculating the transmission ratio of epicyclic gear train and fixed-axis gear train, and the related equations of epicyclic gear train and fixed-axis gear train are listed respectively.
3. Solve the above formula at the same time.
Intermittent motion mechanism:
Non-return claw function: prevent ratchet from reversing.
Kinematic characteristic coefficient of pulley mechanism;
In order to ensure the movement of pulley, the number of grooves of pulley mechanism should be greater than or equal to 3.
Adjustment of mechanical speed fluctuation;
The purpose of adjusting the fluctuation of machine running speed is to make the speed of the machine fluctuate within the allowable range to ensure normal work.
The common method to adjust the periodic speed fluctuation is to add a rotating part-flywheel with large moment of inertia to the machinery.
Moment of inertia of the flywheel mounted on the spindle;
Uneven coefficient of mechanical running speed:
Because J≦☆ and Amax, ωm are finite values, δ is impossible.
If it is "0", even if the flywheel is installed, the running speed of the machine will fluctuate all the time.
The adjustment of non-periodic speed fluctuation can not be adjusted by flywheel, but by regulator.
Balance of rotating parts:
The purpose of balance is to study the distribution and variation law of inertia force, and take corresponding measures to balance inertia force, so as to reduce or eliminate additional dynamic pressure, reduce vibration, improve the working performance of machinery and prolong its service life.
Static balance: the rotating parts can remain static at any position and will not rotate by themselves.
Static equilibrium condition: the resultant force of centrifugal force of each mass on the rotating part is equal to zero.
Dynamic balance: both stationary and moving rotating parts are balanced.
Dynamic balance condition: the resultant force of centrifugal force of each mass on the rotating part is equal to zero, and the coupling distance caused by centrifugal force is equal to zero.
It should be pointed out that the rotating parts with dynamic balance must also have static balance, but the rotating parts with static balance are not necessarily dynamic balance.
For disk-shaped rotating parts, when D/B > 5 (or b/D≤0.2) is usually corrected by static balance test, dynamic balance is not needed. When D/B < 5 (or b/D≥0.2) or rotating parts with special requirements, dynamic balance is generally required.
D- disc diameter b- disc thickness