Resource Assessment of Spiroid Gears Under Variable Loading Conditions

. The brief analysis of reasons of failure of mechanisms and drives of lifting, construction, and road-building machines based on engagement drives, and worm-gear class drives, represented by contact destructions of active surfaces of cog-wheel cogs resulting in malfunctions, breakages, failures, such as wear and furrows, was carried out. The need to create a method for calculating the wear of spiroids with regard of variable loading mode and time was substantiated. The method developed allows, with regard of real modes of operation of handling machinery, equipment and machines, determine the wear intensity values and calculate the spiroid gear drive resource with regard of duration of action of diving torque values in accordance with the set variable loading schedule for lifting, construction, and road-building machines under the wear intensity of the spiroid wheel cog on values of driving torque on the spiroid gear drive output shaft, obtained from experiments.


Introduction
Due to the action of useful resistances, constant or variable in time in the process of operation, occurring on operating equipment, contact destructions of active surfaces of links, wear, furrows on cog side surfaces and fractures, which are the reason of failure of mechanisms of drives of lifting, construction, and road-building machines, form in cog engagements of power transmissions [1]. This type of destruction is typical for quenched surfaces of cog-wheel cogs and is revealed by progressive pitting of material that may reach considerable size, and on work-hardened cog surfaces by flaking lager areas of metal pad [2].
For cog wheels of both open and closed drives, and worm-gear class drives operating in the environment contaminated with abrasive particles, the typical reason of failure is wear and binding of cogs.
The typical feature of worm-gear class drives is the relative slippage at any point of contact of active surfaces of the couple links, the speed value of which increases with the transmission ratio increase.
Mechanisms of drives of lifting, construction, and road-building machines are characterized by intermittent-duty loading modes with the change of drive torques on the gear drive output shaft during the operating cycle time. There modes are characterized by low line speed and related high loads, along with frequent startups and high contacts voltages leading to destruction of the surface of cogs of contacting links and, consequently, wear.
Spiroids belonging to worm-gear type (see figure 1 [3,4,5]), have been applied in various fields of technology for the last fifty years in parallel with worm-gear drives. They became widespread in drives of handling machinery, equipment and machines operating both at constant and variable loading modes. Due to the fact that the considerable number of drives in mechanisms of lifting, construction, and road-building machines is operated at variable loading modes on the operating equipment during the cycle, creating a method of calculation of the spiroid resource by wear with regard of the loading variable nature becomes an actual challenge.
The complexity of building the universal method of calculating gears by wear is in determination of the values of driving torques on the gear drive output shaft during loading cycles and their duration with regard of operating conditions for lifting, construction, and road-building machines [6].
Moreover, the experimental determination of wear intensity of the cog-wheel material is of considerable labour intensity and complexity because it requires the creation of universal laboratory equipment for testing natural and model samples would provide the wide range for changing test conditions approximating the real modes of machines and mechanisms operation. Figure 2 shows experimental dependencies of wear intensity J h and resource L of the spiroid wheel cogs on the values of driving torque on the gear drive output shaft T i . This dependence J h =F(T i ) allows finding wear intensity of the spiroid wheel cog for the corresponding driving torque values T i of the set variable loading schedule.

Research methods
1. Establish the mechanism loading schedule in accordance with the number of ranked relative values of drive torques on the spiroid gear drive output shaft in order of descending and with the relative duration of their action (see figure 3). 2. Calculate main parameters of the spiroid gear drive required for the resource calculation (Table 1) .   Fig. 3. Established schedule of variable loading mechanism [7].
4. Specific calculated force in the engagement corresponding to the driving torque on the gear drive output shaft Т i [8]: whereT i /T max -relative driving torque values determined from the set variable loading schedule; w HEmax -specific calculated force in the engagement corresponding to the driving torque on the gear drive output shaft Т max . 5. Establish the value of the gear drive spiroid wheel cog per the mechanism operating cycle, corresponding to the value of the operating driving torque on the gear drive output shaft Т i and its action duration t i , mm [8]: where J hi -the value of wear intensity of the spiroid wheel material, obtained from experiments, corresponding to the operating driving torque T i (to be determined under the graph, see figure 2); t i -duration of the operating driving torque Т i on the spiroid gear drive output shaft per its operating cycle in accordance with the set variable loading schedule (see figure 3), sec; i=1-umber of engagement couples with the cog-wheel under consideration. 6. Establish the value of the mechanism operation cycles to the maximum wear of the spiroid sheel cogs [h 2 ] [8]:

Results and discussion
Dependence of the wear intensity J h of the spiroid sheel cogs (material: bronze BrА9Zh4) on the values of the driving torque on the spiroid gear drive output shaft Т i was obtained from experiments, allowing to determine the wear intensity and calculate the resource of the mechanism spiroid gear drive with the set variable loading schedule.

Conclusions
The algorithm of calculating the resource of the spiroid gear drive by spiroid wheel wear based on the experimental dependance of the wear intensity on loading torque J h =F(T i ) is given.
Experimental dependence of the wear intensity J h of the spiroid sheel cogs (material: bronze BrА9Zh4) on the values of the driving torque on the spiroid gear drive output shaft Т i was obtained based on bench test results of disc and roller analogy method.