The more common portable air tools have the shaft of the air motor connected to the grinding or cutting tool directly so that the tool is coaxial with the shaft. In order to facilitate more difficult grinding operations, such as the grinding of corners, special air tools known as "angle grinders" are provided having work spindles spaced radially at least several inches from the motor spindle and having positive gear drive means between the motor spindle and the work spindle. These angle grinders have been used for many years.
Heretofore, the drive means for these angle grinders usually consisted of bevel gears which were relatively expensive and required lubrication to avoid excessive wear. A set of such gears could, for example, cost 30 to 40 dollars or more. Because the lubricant is thrown off by centrifugal force at speeds substantially in excess of 8000 revolutions per minute, the gears of these air tools cannot be lubricated properly and wear out rapidly if operated at higher speeds. These tools are inefficient and generally unsatisfactory when used with grinding wheels having an external diameter substantially less than 2 inches.
If, for example, a 1-inch diameter grinding wheel was desired for grinding small diameter fillers, the wheel speed should theoretically be more than 15,000 revolutions per minute for effective grinding, but a portable angle grinder of the type made prior to this invention could not be operated at such speeds without wearing out the gears. The lubricant would be thrown off as fast as it was applied. As a practical matter, the cost of replacing the bevel gears would be prohibitive at such speeds. Operation of the small diameter grinding tools on such angle grinders at lower speeds was not satisfactory because they do not remove metal effectively when operated in that manner.
This problem has existed for more than two decades, and no satisfactory solution was found prior to the present invention. Heretofore, it has been accepted that portable angle grinders should be operated at speeds well below 10,000 revolutions per minute to avoid excessive maintenance costs and should be used with grinding wheels having a diameter greater than 2 inches.
The present invention involves use of an unusual toothed belt drive. Heretofore conventional belt drives have sometimes been used in portable electric tools, such as belt sanders or the like, and they are suitable for use with electric motors because they slip to protect the motor from damage due to sudden stops. In recent years toothed belts have sometimes been used as timing belts for internal combustion engines, but their use for other purposes has been very limited.