During the last 60 years electricians have used offset bending tables published by Jack Benfield. Jack Benfield was a salesman during the late 1930's who had the task of selling EMT to the electrical industry. At that time, one reason for not accepting EMT was the difficulty with bending the tubing. Benfield solved this problem by inventing the Benfield Bender and simplifying the mathematics of bending offsets into simple rules and tables of multipliers avoiding the use of trigonometry. Today these tables are published in just about every handbook and manual on conduit bending and are found in the 2 million books sold as the Benfield Conduit Bending Manual. In his book just below one of his famous Zip Guides on page 12 Jack Benfield states, "The Benfield technique works for all sizes of EMT, rigid conduit, or IMC. The same formulas can be used with power benders on 6 inch conduit...it also works for any make of bender that may be found on any job." The author disagrees with this statement and will tell you why in the following presentation.
The Benfield method over simplifies the bending of offsets by using approximations that have built in errors that increase with the size of pipe, the dimension of the radius, and the steepness of the bend. In fact the author discovered that the Benfield rule of using the cosecant of the angle to multiply times the height of the offset to find the distance between bends (where his table values came from) assumes the pipe has no bends or arcs at all but follows a broken straight line path. The Benfield shrinkage multipliers are constant when in fact the shrinkage and shrinkage multiplier are a function of the centerline bending radius.
These errors have caused the author untold misery over the years resulting in rebending of pipe, banging the pipe on the floor to remove degrees of bend, recutting precut pipe, and sometimes discarding the offset bent pipe entirely. It is a very frustrating task indeed to make every effort to bend an offset to 1/16 of an inch only to find that even with your utmost care in using the Benfield Tables and method that your pipe is anywhere from 3/8 to 2 inches off. What is even more frustrating is thinking that you were the problem and not knowing that the Benfield Tables were the root cause. After all, if these tables have been published for over 60 years and are attached to hand bender handles by manufacturers they must be correct. However, as we shall see, this is not the case.
After developing and installing the equations found in Figures 1 and 2 in an Excel spreadsheet and a JavaScript computer program and evaluating and testing the results, they reveal that, indeed, the Benfield Tables are approximations, the best fit of the least squares or something like that. How can a 1/2 inch bender and a 2, and 4 inch bender all have the same table when they have different bending radii? They don't and that is why those shrinkage values always come out just a little off and become more off as the size of the raceway increases. But with long necked connectors for EMT the errors do not cause too many problems for 1/2, 3/4, and 1 inch EMT for bends up to 30 degrees. But if you use the Benfield Tables and technique for bends over 30 degrees or on 1 1/4 to 6 inch pipe, you are going to have trouble!
The center line bend radii for pipe sizes from 1/2 inch to 5 inches for
71 benders were entered into an Excel spread sheet and used in the calculation of shrinkage and distance between bends and this was compared to the calculated numbers using the Benfield offset formulae given on page 8 of the Benfield Conduit Bending Manual for 10 degrees, 22 1/2 degrees, 30 degrees, 45 degrees, and 60 degrees The spreadsheet performed 3,552 calculations to reveal the following. The errors in shrinkage for a 30 inch high offset varied from 5/16 of an inch for 1/2 inch EMT with a 22.5 degree offset to 3 1/16 inch for 5 inch rigid with a 60 degree offset. The errors in distance between bends for a 30 inch high offset varied from 1/16 of an inch for 1/2 inch EMT with a 30 degree offset to 4 inches for 5 inch rigid pipe with a 60 degree offset. In 11 cases using the Benfield shrinkage values would result in precutting the pipe up to 3 1/16 inches too short. The average error for shrinkage is 9/16 inches and the average error for distance between bends is 2 inches. The results of these calculations and for other offset heights are so overwhelming that the author recommends using the Benfield offset formulae only for 1/2 inch to 3/4 inch EMT with bends no greater than 30 degrees.by Gerald Newton