Investigation of the relation between grinding process parameters and created curvature and roughness of the pushbelt element saddle surface for optimizing the pushbelt element-ring contact in Continuously Variable Transmissions

In a Continuously Variable Transmission (CVT), the pushbelt is the torque transmitting part consisting of around 400 steel elements which are held together by steel ring packs. Relatively high friction losses of the operating pushbelt are associated with the element saddle surface on which the steel rings are lying. The pushbelt runs between two pulleys which are variable in diameter and, therefore, slip between the elements and the steel rings occurs, which leads to these friction losses. In order to increase the efficiency and durability of the pushbelt, the shape and the surface roughness of the element saddle has to be adjusted such that the friction is reduced. The pushbelt element saddle surface is difficult to reach since it is situated in a small slot of the element, and in the production process, it is currently not specially treated after fine blanking. The requirements in order to achieve a friction reduction in the pushbelt's element-ring contact were specified in terms of average surface roughness Ra along the long side of the element saddle, called the horizontal Ra, and the radius across the short side of the element saddle, called the short saddle radius (SSR). A grinding machine was specially built for testing the feasibility of belt grinding for this purpose and to be able to produce pushbelt elements with the required adjustments for pushbelt efficiency tests. The grinding machine uses two grinding belts to process the left and right saddle of the elements which are arranged in a row on a semi-circle in an element holder. In this research the relation between grinding process parameters and created curvature and roughness of the pushbelt element saddle surface was examined. Furthermore, it was investigated which are feasible process conditions in order to achieve the required grinding result, and how accurately the saddle surface characteristics can be predicted in dependency of the machine parameters. For this purpose, machine tests were executed and the ground element saddles were measured with the aid of a white light interferometer in order to evaluate the achieved response variables which are the surface roughness Ra and the SSR. The measurements were analyzed in order to describe the relationship between machine parameters and obtained grinding result. Furthermore, the measurements were used to fit an empirical model to the data, and the applicability of a polynomial and also an exponential model was investigated. In preliminary machine tests, it was found that it is necessary to keep the elements in a certain distance in the element holder in order to achieve a stable grinding result. Furthermore, after the preliminary tests, the grinding belt grit size was specified and it was decided to hold the grinding speed constant during the final experimental design. The chosen predictor variables for the grinding result in the final experimental design were the grinding time and the pressure which is used in the pneumatic cylinder in the machine in order to bring the grinding belt to tension. A full factorial experimental design with nine duplication experiments was used, so in total 18 experiments, where the factors time and pressure were varied on three levels. It was concluded that the applied pressure, so the tension force in the grinding belt, is the dominating factor for the achieved size of the SSR. An increased pressure decreases the size of the achieved SSR and also the variation of the response variables of the samples is decreased at higher pressures. The achieved horizontal Ra on the saddle is affected by both factors, time and pressure, however, the grinding time appears to have more influence. The empirical exponential model and also the polynomial model give a similar prediction for the grinding result within the experimental factor range, but beyond that range, it was found that the exponential model is able to make a better prediction. However, the predicted machine parameters from the exponential model, which would be necessary in order to achieve the desired grinding result according to the requirements, are not feasible. The needed pressure would be too low for the grinding belt to make sufficient contact with the saddle surface and the predicted grinding time would be very high. Within feasible machine conditions, the achieved SSR is always lower than specified in the requirements and decreases rapidly with increasing pressure. Additionally, the lowest roughness Ra which can be achieved according to the exponential model is slightly higher than the requirement. This is consistent with the performed experiments and it was concluded that with the current machine configuration, the requirements cannot be achieved. It is recommended to execute further experiments with an element holder having an increased diameter since with the current holder, although the element's saddle was positioned at the desired radius, the achieved radius across the saddle surface was smaller after grinding. Furthermore, it is recommended to perform further experiments with different kinds of grinding belts having finer grain sizes for achieving a lower surface roughness or having structured abrasives for less variation and a better predictability in the grinding result.Mechanical Engineering | BioMechanical Desig