Helical gear refers to the category of mechanical. The main purpose of this operation is the conversion of rotational motion into translational or vice versa. This type of transmission consists of only two elements - a screw and a nut.
Device description
As already mentioned, the helical gear mechanism is used to convert motion. The most common examples of using this system are devices such as jacks, presses, metal cutting machines, rolling mills, lifting equipment, etc. It is also worth noting that all these are examples of converting rotational motion into translational motion. But for the reverse procedure, this device is used very rarely. For example, the mechanism for moving the film of the camera works according to the inverse principle of motion conversion.
There are several advantages of this system: silent operation, smooth engagement, simplicity of design, the possibility of obtaining great power.
However, there are a number of drawbacks: quite often the helical gear sticks, and its efficiency, that is, efficiency, is low.
Device and types
Currently, there are two main devices of the system. Its first type contains a fixed nut and a movable screw, while the second type, on the contrary, has a movable nut and a fixed screw. The screw category can be attributed to the first category of devices, and the second group is used, for example, in machine spindles and in other devices.
There are also several types of helical gears:
- Slip system.
- A rolling system, characterized in that the nut has grooves in which the balls are placed.
- Planetary roller gears, which are considered quite promising, as they are characterized by high accuracy and rigidity.
- The wave form of transmission, it is characterized by rather small translational movements.
- Hydrostatic helical gear, characterized by a low degree of friction, low wear and fairly high accuracy.
Thread and calculation
In addition to the fact that there are several types of systems, there are also several types of threads for nuts and screws. If it is necessary to ensure the least friction between the parts, then a rectangular view is used. However, it is very important to note that the manufacturability of this type of connection is rather low. In other words, it is impossible to cut such a thread on a thread milling machine. If we compare the strength of a rectangular and trapezoidal thread, the first loses significantly. Because of this, the distribution and use of rectangular threads in helical gears is severely limited.
For these reasons, the trapezoidal thread has become the main type that is used to mount the transmission screws. That type has three types of steps - small, medium, large. The most popular was the mid-pitch system.
The calculation of the helical gear is reduced to the calculation of the gear ratio. The formula is as follows: U = C / L = pd / pK. C is the circumference, L is the screw stroke, p is the screw pitch, K is the number of screw strokes.
Ball Screw Transmission (Ball Screw)
Ballscrew - this is one of the varieties of linear drive, which also serves to convert rotational motion into translational. However, there is a difference, which consists in the fact that this type of system is characterized by very low friction.
The role of the screw in such systems is played by the shaft, which is usually made of very strong steel. On its surface, this device has treadmills with a specific shape. It is this device that can interact with the nut. However, their work is not carried out directly, as is the case in a conventional helical gear, but through small balls. The principle of rolling friction is used here.
This principle of interaction provides very high rates of performance (COP), as well as high overload characteristics.
Application and development of ballscrews
Ball screw transmission is most often used in an industry such as aircraft construction, in rocket science to move steering surfaces, in vehicles. The widest range of use of such a system can be observed in precision engineering, in particular, in CNC machines.
The history of the creation of such a screw is rather unusual, since the very first most accurate ball screw was obtained using a low-current conventional screw. The device was as follows: a small structure of several nuts, tensioned by a spring, was mounted on the screw, after which it was rubbed along the entire length.
There was an opportunity to average the pitch errors of both the screw and the nut by moving the elements along the base, as well as by changing the direction of tension.
Using ballscrew
In order to achieve a long service life for the ball screw, it is necessary to follow the operating rules of this system. In order for her to be able to maintain all her indicators at the proper level, including accuracy, it is very important to monitor the cleanliness of the working space of the device. Abrasive particles such as dust, chips, etc. should not fall onto a working pair.
Most often, such problems are solved by the fact that a screw with a nut is installed with corrugated protection made of rubber or polymer materials. This completely eliminates the possibility of contamination. If the system works in open mode, then this problem can be solved in another way. In such cases, a compressor is mounted, which at high pressure delivers purified air to the working steam.
Since the system works on the principle of rolling friction, there is the possibility of preload, which allows you to remove unnecessary play backlash. Backlash is the gap that forms between the rotational and translational motion at the moment when it changes its direction.
Transmission quality
Like any other system, this one has its advantages and disadvantages.
The disadvantages of the device include the fact that there is a chance of a reverse gear if the angle of operation of the ball screw is too large. This is due to the fact that the friction is too low, and therefore the nut does not block when lifting. It transfers linear force to torque. In addition, the use of such transmission systems on hand-held devices is not recommended.
The advantages include the fact that a low percentage of friction causes a low dissipation, which, in turn, greatly increases the efficiency of the entire system. In terms of this indicator, ballscrew is superior to any other analogue of a transmission that is engaged in converting rotational motion into translational motion. The maximum coefficient of performance for the most common ball screws exceeds 90%. For comparison, we say that the closest metric or helical gears have an efficiency of maximum 50%.
Due to the fact that there is practically no sliding in a ball screw, this has a positive effect on increasing the service life of ballscrews and on efficiency, since the downtime during repair, lubrication or replacement of parts is significantly reduced. Therefore, such devices are the most profitable.
Manufacture and precision
The most high-precision screws for ballscrews can only be obtained by grinding the material. There is another way to get a screw - this is knurling. The cost will be significantly lower than when grinding, but the product error will be about 50 microns per 300 mm stroke. Note that the most highly polished parts are characterized by an error of 1-3 microns per 300 mm, and some are even smaller. To get a blank for a future screw, the material must go through a rough machining process, after which it is hardened and ground to the required state.
The instrumental view of ballscrews most often has an accuracy of up to 250 nm per centimeter. To make such products, it is necessary to go through the process of milling and grinding. It is necessary to carry out these operations on very high-precision equipment. The feedstock for such screws is Invar or Invar alloys.