Worm drive:
Worm drive is a kind of transmission that transmits motion and power between two axes that are interlaced in space. The angle between the two axes can be any value, and the commonly used one is 90°.Worm drives are used to transmit motion and power between staggered shafts.
1 Introduction
The worm drive is composed of a worm and a worm gear, and the worm is generally the active part.Worms, like threads, can be divided into right-handed and left-handed worm drives, which are called right-handed and left-handed worms, respectively.The worm with only one helix is called a single-head worm, that is, the worm rotates once, and the turbine turns one tooth. If there are two helixes on the worm, it is called a double-head worm, that is, the worm rotates once, and the turbine turns two teeth. .
2. Features
(1) Large transmission ratio and compact structure.The number of worm heads is represented by Z1 (generally Z1=1~4), and the number of worm gear teeth is represented by Z2.It can be seen from the transmission ratio formula I=Z2/Z1 that when Z1=1, that is, the worm is single-headed, and the worm must turn Z2 to turn the worm wheel for one turn, so a large transmission ratio can be obtained. Generally, in power transmission, take The transmission ratio I=10-80; in the indexing mechanism, I can reach 1000.Such a large transmission, such as gear transmission, needs to adopt multi-stage transmission, so the worm transmission has a compact structure, small size and light weight.
(2) The transmission is stable and there is no noise.Because the worm tooth is a continuous and uninterrupted helical tooth, it is continuous when meshing with the worm gear teeth, and the worm tooth does not enter and exit the meshing process, so the work is stable, and the impact, vibration, and noise are relatively small
(3) It is self-locking.When the helix angle of the worm is small, the worm can only drive the worm gear, but the worm gear cannot drive the worm to rotate.
(4) The efficiency of worm transmission is low. It is generally believed that the efficiency of worm transmission is lower than that of gear transmission.Especially for self-locking worm drives, the efficiency is below 0.5, and the general efficiency is only 0.7-0.9.
(5) The calorific value is large, the tooth surface is easy to wear, and the cost is high.
3. Cylindrical worm
Cylindrical worm drive is a worm drive in which the worm indexing surface is a cylindrical surface.
Among the worm drives, Archimedes cylindrical worm drives and arc-contract worm drives are commonly used.① The end tooth profile of the Archimedes worm is an Archimedes spiral, and its axial tooth profile is a straight line.The Archimedes worm can be processed on a lathe with a trapezoidal turning tool, so it is simple to manufacture, but it is difficult to grind, so the precision is not high.In the Archimedes cylindrical worm drive, the angle between the contact line of the worm and the tooth surface of the worm gear and the relative sliding speed is very small, and it is not easy to form a lubricating oil film, so the bearing capacity is low.② Spiral cylindrical worm transmission is a worm transmission in which the tooth profile of the worm shaft surface (or normal surface) is a concave arc and the tooth profile of the worm wheel is a convex arc.In this kind of transmission, the angle between the contact line and the relative sliding speed is large, so it is easy to form a lubricating oil film, and the convex and concave tooth profiles are meshed, the equivalent curvature radius of the tooth profile on the contact line is large, and the contact stress is low. Its load capacity and efficiency are higher than other cylindrical worm drives.
4. Main parameters
The parameters and geometric dimensions of various cylindrical worm drives are basically the same.It is the main parameter of Archimedes cylindrical worm drive.The plane passing through the axis of the worm and perpendicular to the axis of the worm gear is called the middle plane.On the middle plane, the [1] tooth profile of the worm is a straight line, and the tooth profile of the worm wheel is an involute, and the meshing of the worm and the worm wheel is equivalent to the meshing of the rack and the involute gear.Therefore, the calculation of the parameters and geometric dimensions of the worm drive is roughly the same as that of the gear drive, and the parameters and dimensions on the mid-plane are used as the benchmark in design and manufacture.
5. Worm drive
The axial pitch pX of the worm should be equal to the pitch pt of the end face of the worm gear, so the axial modulus of the worm should be equal to the modulus of the end face of the worm gear, expressed in m, and m should be taken as a standard value.The axial pressure angle of the worm should be equal to the end face pressure angle of the worm wheel, represented by α, usually the standard pressure angle α=20°.
The worm is equivalent to a helix, and its helix is also divided into left-handed and right-handed, single-headed and multi-headed.Usually the number of heads of the worm Z1=1～4, the more the number of heads, the higher the efficiency; but if the number of heads is too many, such as Z1>4, the indexing error will increase and it is not easy to process.The number of teeth of the worm gear Z2=iZ1, i is the transmission ratio of the worm drive, i=n1/n2=Z2/Z1.For worm drives that generally transmit power, Z2=27～80.When Z2<27, the worm gear teeth are prone to undercutting; and when Z2 is too large, the bending strength of the worm gear teeth may be insufficient.Let d1 represent the diameter of the worm indexing circle, then the helix angle λ on the worm indexing cylinder can be obtained by the following formula. Introducing q=Z1/tgλ in the above formula, the diameter of the worm indexing circle can be obtained as d1= qm.In the formula, q is called the characteristic coefficient of the worm.In order to limit the number of hobs, the standard specifies the q value that matches each modulus.Usually q=6～17.Worm gear indexing circle diameter d2=Z2m.
6. Failure form
In worm drives, worm gear teeth fail in the form of pitting, wear, binding and tooth bending.However, the general worm transmission efficiency is low, the sliding speed is large, and it is easy to generate heat, etc., so glueing and wear damage are more common.
In order to avoid gluing and reduce wear in worm drive, the material of worm drive must have the properties of anti-friction, wear resistance and gluing resistance.Generally, the worm is made of carbon steel or alloy steel. The surface of the spiral should be heat treated (such as quenching and carburizing) to achieve high hardness (HRC45~63), and then it should be ground or honed to improve the bearing capacity of the transmission.Most worm gears are made of bronze, and sometimes brass or cast iron are used for transmissions that are not important for low speeds.In order to prevent gluing and slow down wear, a good lubrication method should be selected, and a lubricating oil containing anti-gluing additives should be selected.There are no well-established calculation methods for gluing and wear in worm drives.Tooth surface contact stress is an important factor causing tooth surface gluing and wear, so the calculation of tooth surface contact strength is still the basic calculation of worm drive.In addition, sometimes the bending strength of the gear teeth should be checked.Generally, the worm teeth are not easily damaged, so it is usually not necessary to calculate the strength of the teeth, but the strength and stiffness of the worm shaft should be checked if necessary.Heat balance calculations should also be carried out for closed drives.If the heat balance calculation cannot meet the requirements, add a heat sink outside the box or use a forced cooling device.
7. Introduction to structure
Generally, the worm and the shaft are made into one body, which is called the worm shaft.The structure of worm gear can be divided into three forms of worm drive.① Integral type: used for cast iron and bronze worm gears with small diameters.② Ring gear press fit: the hub is cast iron or steel, and the rim is bronze.③Bolt connection type: the wheel rim and the hub adopt reamed holes and are connected by bolts. This structure is easy to assemble and disassemble.
8. Transmission application
Worm drive is often used in occasions where the two shafts are interlaced, the transmission ratio is large, the transmission power is not too large, or intermittent work.
When the worm drive is required to transmit a large power, in order to improve the transmission efficiency, Z1=2~4 is often taken.In addition, since the transmission is self-locking when γ1 is small, it is often used in hoisting machines such as hoists for safety protection.It is also widely used in machine tools, automobiles, instruments, metallurgical machinery and other machines or equipment. The reason is that the use of axle movement can reduce the consumption of force, so it is widely promoted.
