TEETH GEOMETRY AND CONTACT PRESSURE CALCULATION OF EXTERNAL CYCLOIDAL GEARS

Cycloidal (also called epicyclical or convex-concave) gears are used less often than common involute gears, which are very easy to manufacture and can be modified by corrections to the gear profile. Cycloidal gears are very sensitive to the proper axial distance between the pinion and the gear. The main advantage of convex-concave gears is the lowering of the contact pressure due to teeth flanks meshing and also the lowering of the slide ratios compared to involute gears. The calculation of the selected geometrical parameters and the contact pressure between the teeth flanks of the cycloidal gearing is described in the presented article.


INTRODUCTION
The mathematical model of convex-concave gearing is the basis of the geometry model calculation and as described in detail in [1]. The determination of the geometric parameters in the gear's teeth flanks is based on the equations evaluated from the shape of the path of contact. The general path of contact starting at point A and ending at point E for this type of gearing is presented in Figure 1. The arcs of the path of contact are circular arcs defined by their radii rkh for the upper one and rkd for the lower one. The centres of the arcs Skh and Skd, which lie on the common link passing through the contact inflection point C, are defined by the coordinates xSkh, ySkh and xSkd, ySkd in the coordinate system with the origin located in contact point C.
Points A and E are limiting points of the teeth gear mesh. Their position can also be projected onto the teeth flanks' cycloidal curves in both meshing gears, which limits the working area of the teeth flanks [2,3].

GEOMETRY OF THE TEETH FLANKS
The cycloidal teeth can generally be understood as any teeth whose tooth flank forms a curve with a convex and a concave part. Such teeth are present when the contact path is a socalled S-curve, as defined above [4]. Deriving the form of the correctly mating profiles of a cycloidal gearing can be done using basic knowledge of differential geometry and the direct application of the fundamental law of gearing [5]. The main goal of this method is to determine the relation between the pressure angle at various points of the path of contact α and the angle of the gear rotation between pressure angles of two arbitrary points φr (α) (Figure 2). Teeth geometry and contact pressure calculation of external cycloidal gears 29. ( The x and y coordinates are defined for the coordination system with the origin aligned to the point of rotation of the pinion O1 and the gear O2. The upper signs in the equations stand for the upper part of the path of contact (indexed with h) and the lower signs in the equations stand for the lower part of the path of contact (indexed with d).
The division of the contact path into an upper and a lower part requires the division of all geometric and other cycloidal gear pair parameters into analogous parts, which will be defined according to the corresponding parts of the contact path curve [6].
It is suitable to derive Equations 2 and 3 into a form that defines the addendum (indexed with a) and the dedendum (indexed with f) of the gear teeth separately.

Pinion
The coordinates of the pinion 1 addendum flank curve are based on the upper part of the contact path arc dimension, according to the following equations: The coordinates of the pinion 1 dedendum flank curve is based on the lower part of the contact path arc dimension, according to the following equations:

External gear
The tooth flank profile coordinates of the external gear 2 can be derived from the equations defined for the pinion 1 considering the gear ratio between them, which is defined as: The coordinates of the external gear addendum flank curve are based on the lower part of the contact path arc dimension, according to the following equations: The coordinates of the external gear dedendum flank curve is based on the upper part of the contact path arc dimension, according to the following equations:

SINGLE MESH POINTS
The coordinates of the single mesh points B and D are obtained by solving Equation 1, while considering the angle turns φrAD and φrEB to be equal to the angle defined by the pinion tooth pitch [1].
Teeth geometry and contact pressure calculation of external cycloidal gears 31.
The pressure angles αB and αD in the single mesh points B and D are calculated using the following transcendental equations: The single mesh points are important for the definition of the normal force value, which is divided between two pairs of meshing teeth at the path of contact curves AB and CD ( Figure 1).

CONTACT PRESSURES
The contact pressure calculation is based on Hertz contact theory [3,7], which is also defined for the upper as well as the lower parts of the path of contact by the following equations: The normal forces at the contact points A to C (F1h) and at C to E (F1d) are calculated at the pinion 1, loaded by the input torque Mk1, as follows: The reduced Young's modulus of the pinion and gear material is a part of the material coefficient ZE, which is calculated by Equation 8 [9]. The reduced radius of curvature ρred is calculated according to these equations: The radii of curvature of the pinion 1 addendum (a) and dedendum (f) are defined as: The radii of curvature of the gear 2 addendum and dedendum are defined as:

CALCULATION OF SELECTED GEAR PAIR VALUES
The selected gear pair with the module m = 4 mm and the teeth number z1 = 16 and z2 = 24 will represent the application of all derived equations into a model of cycloidal gear pair geometry and the distribution of contact pressure by meshing of the gear teeth.
The geometry is influenced by the module m, the number of teeth z, the radii of the contact path arcs rkh and rkd, and the pressure angle at the point C αC. The convex-concave condition is satisfied, if there is a valid inequation [1].
The radii of the contact path arcs in the symmetric arrangement within the selected gear pair were defined as rkh = rkd = 8 mm and the pressure angle in the point C as αC = 20°, which satisfies the inequation (28). The geometry of the selected gear pair is presented in Figure 3.
The contact pressure between the pinion and the gear at the tooth flanks is calculated by the unit values of the torque Mk1, the speed w1 and the gear tooth flank width l, all of which are defined as being equal to 1. The gears are considered from steel with Poisson's ratios μ1 = μ2 = 0.3 and Young's moduli E1 = E2 = 210000 MPa. The Hertz pressure distribution, as projected onto the pinion and gear teeth flanks, is presented in (Figure 4).
The contact Hertz pressure pH between the pinion and the gear, up to the angle of the pinion rotation between the pressure angles of two arbitrary points φr1 (α), is shown in Figure 5.

CONCLUSION
The article presents a possible approach for modelling cycloidal gear teeth flanks based on the path of contact curves. The calculation of the maximum contact pressure at various points of the gear pair teeth flanks is also defined. The calculation of a selected gear pair is performed by the unit values of the torque, speed and tooth flank width. The obtained model is fully parametric and allows us to calculate the Hertz pressures for various combinations of the characteristic gear pair values, such as the module, the teeth numbers, the pressure angle in the contact point C and the radii of the path of contact curve. The change in the characteristic gear pair values enables us to pursue further research on their influence on Hertz pressure values [9,10,12].