Screw transmission belongs to the category of mechanical. The main purpose of this operation is to convert 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 screw drive mechanism is used to convert motion. The most common examples of the use of this system were such devices as jacks, presses, metal-cutting machines, rolling mills, lifting equipment, etc. It is also worth noting that these are all examples of converting rotational motion into translational. But for the reverse procedure, this device is used very rarely. For example, the mechanism for moving the film of a camera works on the reverse principle of motion conversion.
There are several advantages of this system: silent operation, smooth engagement, simple design, large force can be obtained.
However, there are also a number of disadvantages: quite often the helical gear sticks, and its efficiency, that is, the efficiency, is low.
Device andspecies
Currently, there are two main units of the system. Its first type contains a fixed nut and a movable screw, and the second type, on the contrary, has a movable nut and a fixed screw. The first category of devices includes a screw jack, and the second group is used, for example, in the lead screws of machine tools and other devices.
There are also several types of screw gears:
- Sliding system.
- A rolling system characterized by the fact that the nut has grooves in which balls are placed.
- Planetary roller gears, which are considered quite promising, as they are distinguished by high accuracy and rigidity.
- Wave type of transmission, it is distinguished by rather small translational movements.
- Hydrostatic screw transmission featuring low friction, low wear and fairly high accuracy.
Carving and calculation
Besides there are several types of system, there are also several types of threads for nut and screw. If it is necessary to ensure the least friction between parts, then a rectangular view is used. However, it is very important to note here 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 rectangular and trapezoidal threads, then the first loses significantly. Because of this, the distribution and use of rectangular threads in screw drives is severely limited.
For these reasons, the main type that is used for the device of transmission screws has become trapezoidal thread. That type has three types of step - small, medium, large. The most popular is the medium pitch system.
Calculation of the screw transmission is reduced to the calculation of the gear ratio. The formula looks like this: U=C/L=pd/pK. C is the circumference, L is the lead of the screw, p is the pitch of the screw, K is the number of turns of the screw.
Ball Screw (BSC)
Ball screw - this is one of the varieties of linear drive, which also serves to convert rotational motion into translational. However, there is a difference here, which is that this type of system is characterized by very little friction.
The role of the screw in such systems is performed by the shaft, which is usually made of very strong steel. On its surface, this device has treadmills with a specific shape. It is such a device that is able to interact with the nut. However, their work is not carried out directly, as is the case in a conventional screw gear, but through small balls. It uses the principle of rolling friction.
This principle of interaction provides very high coefficient of performance (COP), as well as high overload characteristics.
Application and development of ball screws
Ball screw is most often used in industry such as aircraft manufacturing, in rocket science to move rudderssurfaces 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 this screw is rather unusual, as the very first most accurate ball screw was obtained using a low-precision conventional screw. The device had the following appearance: a small structure of several nuts, tensioned by a spring, was mounted on the screw, after which it was lapped along the entire length.
It became possible 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 tension direction.
Using ball screws
To achieve a long service life of the ball screw, it is necessary to follow the operating rules of this system. In order for it to be able to maintain all its indicators at the proper level, including accuracy, it is very important to monitor the cleanliness of the working space of the device. The operating steam must not be exposed to abrasive particles such as dust, chips, etc.
Most often, such problems are solved by installing corrugated protection made of rubber or polymer materials on the screw with the nut. 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 installed, which, under high pressure, supplies purified air to the working steam.
BecauseSince the system works on the principle of rolling friction, it becomes possible to preload, which allows you to remove unnecessary gear play. Backlash is the gap that forms between 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 for reverse transmission if the angle of operation of the ball screw is too large. This occurs because there is too little friction so that the nut does not lock when lifted. It transfers linear force into torque. In addition, the use of such transmission systems on handheld devices is not recommended.
The advantage is that the low percentage of friction causes low dissipation, which, in turn, greatly increases the efficiency of the entire system. According to this indicator, the ball screw is superior to any other analogue of the transmission, which is engaged in the transformation of rotational motion into translational. The maximum efficiency for the most common ball screws exceeds 90%. For comparison, let's say that the closest metric or helical gears have a maximum efficiency of 50%.
Because there is practically no slip in the ball screw, it has a positive effect on increasing the service life of the ball screw and on the economy, since downtime for repairs, lubrication or replacement of parts is significantly reduced. Therefore, these devices are the mostprofitable.
Fabrication and precision
The most high-precision ball screw screws 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 grinding, but the product error will be about 50 microns per 300 mm stroke. Note that the most high-precision ground parts are characterized by an error of 1-3 microns per 300 mm, and some even less. To obtain a blank for the future screw, the material must undergo a rough machining process, after which it is hardened and ground to the required state.
The instrumental view of the ball screw 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 raw material for these screws is Invar or Invar alloys.
