Main parameters of ball screw
When it comes to the selection of ball screw, it is necessary to talk about its common parameters first, and then to determine its model from these parameters.
1. Nominal diameter
That is, the outer diameter of the lead screw, the common specifications are 12, 14, 16, 20, 25, 32, 40, 50, 63, 80, 100, 120, but please note that in these specifications, the manufacturers generally only have 16~50 goods, that is to say, most of the other diameters are futures (see single production, the delivery period is about 30~60 days, It takes about 2~2.5 months for Japanese products and 3~4 months for European and American products).
Nominal diameter and load are basically proportional, the greater the diameter of the load, the specific value can be consulted by the manufacturer's product sample. There are only two concepts here: dynamic load rating and static load rating. The former refers to the rated axial load under the state of motion, and the latter refers to the rated axial load under the state of rest. Refer to the former when designing. It should be noted that the rated load is not the maximum load, and the smaller the ratio of actual load to rated load, the higher the theoretical life of the lead screw. Recommended: The diameter should be 16 to 63.
Ball screw, according to the domestic classification, accuracy grade P1, P2, P3, P4, P5, P7, P10, Japan, South Korea, Taiwan Province of China using JIS grade, namely C0, C1, C2, C3, C5, C7, C10; The standard used in European countries is IT0, IT1, IT2, IT3, IT4, IT5, IT7, IT10.
Generally, the ball screw purchased by our company is from Taiwan, which has a relatively high cost performance, followed by Japan.
The precision expression method is: No matter how long your ball screw is, take any section of 300mm, the error is within the precision represented by the grade. The precision represented by each grade is as follows:
In general, general machinery USES the C7, C10, numerical control equipment, generally using C5, C3 level (C5 is more, most of domestic nc machine tools are C5), aviation manufacturing equipment, precision projection and C3 commonly used three coordinates measuring equipment, precision of C2 (as far as I know, the domestic civilian goods market can't buy more than C3 ball screw).
In addition, C7, C10 class is generally made by rolling method, AND C5 class and above are made by grinding method.
To sum up, non-standard design commonly used ball screw accuracy grade C7 (rolling method manufacturing or some people called manufacturing), and the ball screw accuracy grade has higher requirements, C5 (grinding method manufacturing) is also enough. Of course, we should make a case by case analysis.
3. Preloading grade
Also called preloading, about preloading, we do not need to understand the specific preloading force and preloading way, only need to choose the preloading grade according to the manufacturer's sample. The higher the preload grade, the tighter the nut and screw fit; Conversely, the lower the grade, the looser.
The principle to follow is: large diameter, double nuts, high precision, large driving torque, when the application of the screw appears above the situation, the prepressure grade can choose a higher point, on the contrary, choose a lower point.
Length has two concepts, one is full length, the other is thread length. Some manufacturers only calculate the full length, but others need to provide thread length. Thread length also has two parts, one is the full length of thread, one is the effective stroke. The former refers to the total length of the thread part, the latter refers to the theoretical maximum length of the nut linear movement, thread length = effective stroke + nut length + design margin (if you need to install protective cover, but also consider the length of the compressed protective cover, generally 1/8 of the maximum length of the protective cover calculation).
In the design drawing, the length of the screw can be roughly according to the parameters of the stack: screw length = valid travel allowance + + design + nut length on both ends of the supporting length (bearing width + + lock nut width margin) + power input connection length (if using coupling is roughly half the length of the coupling + allowance). In particular, if your length is too long (more than 3 meters) or the length-diameter ratio is large (more than 70), it is best to consult the sales staff of the manufacturer in advance whether the production can be made. The overall situation is that the maximum length of the conventional products of domestic manufacturers is 3 meters, and the special products are 16 meters. The conventional products of foreign manufacturers are 6 meters, and the special products are 22 meters. Of course, it is not to say that domestic manufacturers cannot produce longer, but the price of fixed products is quite outrageous. Recommended: length as far as possible to choose 6 meters below, more than the rack and pinion more cost-effective.
4, nut form
There are many types of nuts on product samples from various manufacturers, and the first few letters in the general type indicate the type of nut. According to the flange form, there are about round flange, single cut flange, double cut flange and no flange. According to the length of the nut is divided into single nut and double nut (note, single nut and double nut no load and rigidity difference, this point do not listen to the factory sales staff speech, single nut and double nut main difference is the latter can adjust the prepressure and the former can not, in addition, the price and length of the latter are roughly 2 times the former). When the installation size and performance allow, the designer should try to choose the conventional form in order to avoid the delivery of spare parts during maintenance. Recommended: Double nuts for frequent operation and high precision maintenance, and single nuts for other occasions. Recommended: Use inner circular double cut flange single nut as far as possible.
After understanding the main parameters of the screw, we can select the type according to our own requirements.
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