Abstract
A cycloidal speed reducer employing gears with permanent magnets acting as teeth is described. The magnets, which have their axes radially oriented in both the orbiting gear and the fixed internal gear, are inserted in holes drilled in nonmagnetic rims without protruding from the cylindrical exposed surfaces. Because the orbiting gear is not restrained radially, it contacts the fixed gear and rolls on its inner surface. A normal force is developed at the contact point between the gears to balance the magnetic attraction and the centrifugal force of the orbiting gear. The friction available due to this normal force increases the transmission’s torque capacity, which is further increased by elimination of the gap between the gears. Also, the radial load on the supporting orbiting gear bearing is eliminated. A prototype with a reduction ratio of 26 is being tested.
1. Introduction
Speed reducers based on the magnetic transmission of forces instead of on conventional mechanical elements such as gears, chains and belts, are becoming increasingly attractive. In addition to offering the advantages of quiet operation, sustainment of overloads without damage, and not requiring lubrication, their efficiency and load capacity in theory appear not to be very far behind those of mechanical gearboxes [1,2]. Speed reducers employing “gears” having radially oriented permanent magnets in alternating polarities are described in a number of patents [3-6]. In other magnetic transmissions, permeable iron elements are used to guide a magnetic field of alternating direction resulting from the rotation of a central wheel with permanent magnets of alternating polarities causing a slow rotation of an exterior ring having a larger number of permanent magnets. Such is the case in the transmissions described in [1], and [7]. In reference [2], a magnetic cycloidal transmission with a topology similar to that of the speed reducer of this paper, is analyzed.
In the transmissions treated in all the previously mentioned references, the magnetic forces of attraction act across small gaps between the elements of the mechanism with no contact between them. In contrast with this characteristic, the magnetic gears of the cycloidal speed reducer treated in this paper contact one another due to the fact that the orbiting gear is free to move outwardly under the action of the magnetic attraction from the fixed gear and the centrifugal force. The advantages of this concept are pointed out in the description that follows. As will be seen, another feature incorporated in the speed reducer of this paper is a novel method of balancing the orbiting gear. This speed reducer is referred to as the UNAM cycloidal magnetic speed reducer.
2. Description of the UNAM Cycloidal Magnetic Speed Reducer
Figure 1 shows longitudinal and transverse cross sections of the UNAM cycloidal magnetic gear speed reducer. Each gear has an even number of radially mounted bar magnets distributed uniformly along its periphery in alternating polarities. A pair of N, S poles corresponds to a tooth of a conventional gear. The magnets, which are inserted in radial holes drilled in rims made of non magnetic stainless steel, do not protrude from the rims’ exposed cylindrical surfaces. The rim of the fixed gear is press fitted onto an outer ring made of permeable steel, and the rim of the orbiting gear is press fitted onto a central permeable steel disk. In cycloidal speed reducers, the orbiting gear is driven by a crank of the input high speed shaft. Because the speed ratio of the prototype magnetic unit built is rather large, 26:1, which would require a small crank radius, it was possible to omit a crank as such and use instead a tongue and groove arrangement as