The study of the use of multi-disc safety friction clutches in the working bodies of crank presses

Studies of safety couplings have shown that when the friction forces in splined joints of friction disks and half couplings are taken into account, the moment of friction of the clutch increases compared to the moment calculated without taking these forces into account. Among the safety clutches, friction clutches occupy a special place in use in crank presses due to the increased accuracy of limiting the transmitted load. This allows you to increase the efficiency of protection of parts and drive units of machines, as well as reduce their weight and dimensions. Friction disks are pressed against each other with the force created by the spring, and the moment of friction arising between them ensures the connection of the leading parts with the followers. The friction force in the slots significantly reduces the transmitted force of pressing on subsequent disks, but its influence is not taken into account in existing studies.


Introduction
Studies of safety couplings have shown that when the friction forces in splined joints of friction disks and half couplings are taken into account, the moment of friction of the clutch increases compared to the moment calculated without taking these forces into account.
Among the safety clutches, SFC friction clutches occupy a special place due to the increased accuracy of limiting the transmitted load. This allows you to increase the efficiency of protection of parts and drive units of machines, as well as reduce their weight and dimensions.
Friction disks are pressed against each other with the force created by the spring, and the moment of friction arising between them ensures the connection of the leading parts with the followers. The friction force in the slots significantly reduces the transmitted force of pressing on subsequent disks, but its influence is not taken into account in existing studies [1][2][3][4][5].
Previous studies have shown that when the friction forces in splined joints of friction disks and half couplings are taken into account, the moment of friction of the clutch increases compared to the moment calculated without taking these forces into account.

Figure 1.
Multi -disk chart SFC Consider the multi-disc chart SFC presented in Figure 1. The coupling consists of coupling halves 1 and 2, a package of friction disks 3 and 4, a thrust disk 5, a pressure disk 6, a control device and a pressure spring 7. The disks 3 are connected to the drum of the coupling half 2, and the disks 4 to the hub of the pressure disk, which is mounted on the hub of the coupling half 1 with the possibility of rotation relative to it. The thrust disk 5 is rigidly mounted on the hub of the coupling half 1.
The control device consists of rolling elements 8, which are placed with a gap in the sockets ( Fig.  1, section AA) of the thrust and pressure plates.
When SFC is operating, press forces 6 act on n F − spring pull force 7, о F − circumferential force and p F − spacer (squeezing) force ( Fig. 1, section AA). Force action p F , as follows from fig. 1, reduces the force of pressing against each other by friction pairs 3-4. As the friction pairs move away from the support flange of the pressure plate, the action of the spacer force decreases due to the influence of friction forces t F , t1 F .

Methods
Consider two cases: 1. Friction in the elements of the connection of the disks with the leading and driven parts is not taken into account.
The moment of friction forces SFC is determined by the formula: 2. Subject to the consideration of friction in splined joints. Let us consider successively the transfer of the friction moment from the leading disks to the driven disks, shown in Fig. 1.
The moment of friction forces of the friction group of the clutch is equal to:

Results
Spacer force is determined by the formula: Substituting expressions (1) and (2) into equation (3), we obtain: Torque 1 T can be imagined as F − normal force acting on the first friction working surface from the support flange of the pressure plate: F − friction force in spline joints of the pressure plate: f − coefficient of friction between the working surfaces of the spline connection; H R − outer radius of friction disk 4; F − circumferential force transmitted by the friction disk: Given relation (6), we obtain [6-9]: The moment of friction, perceived by the second disk (the first driven disk) F , acting on the second pair is determined by the formula Using expressions (8) and (9), we write the spacer force 1 z F for an arbitrary odd pair 1 z : The total moment of friction forces SFC is z Т − moment of friction forces of one friction pair.
We write expression (12) in the following form: In view of relations (11) and (12), expression (13) can be written as We introduce the following notation: Then expression (12), taking into account formula (11), can be written as The geometric relationships of friction discs are found based on the following reasoning. The outer, middle and inner radii of the friction disk are interconnected by the ratio: ν − disk width ratio.
The second ratio, connecting the outer, middle and inner radii of the friction disk used in the calculations, has the form: Solving the system of equations (1) and (4), we find: Usually accepted ν =0,20, then, taking into account relations (18) and (19), we find:   -SFC torque with regard to friction in splined joints at = 10 is approximately 2 times higher than without friction (Fig. 2); Accuracies the accuracy coefficient SFC, taking into account friction in splined joints, increases with the number of friction pairs of the friction group, asymptotically approaching the value of the accuracy coefficient of the safety friction clutch without the control device (Fig. 3, curve 2); -with a minimum number of friction pairs ( z =2) indicators of load capacity and SFC response accuracy taking into account friction in spline joints and without taking into account friction are close to each other, which is explained by the minimal influence of friction forces in spline joints.
Thus, the study found that taking into account the frictional forces in the spline joints of friction disks with half couplings in multi-disc SFCs allows one to obtain refined dependences of the torque and accuracy coefficient on the number of friction pairs.
With a large number of friction pairs, the torque of multi-disk SFCs, taking into account friction in spline joints, significantly (two or more times) exceeds the torque without taking into account friction in spline joints.
Accounting for friction in splined joints significantly (up to 4 times) increases the accuracy coefficient of multi-disk SFC with a large number of friction pairs.