Calculating Door Weight
Accurate door weight is important
By Tim Kolhoff
Many service calls require correcting an improperly balanced overhead door that employs torsion springs. In a previous article (PDD May 2005), the importance of cable length was discussed as it pertains to proper operation of sectional overhead doors. Here, another important factor in counterbalancing theory will be discussed: door weight. A door can be weighed if a scale is available, or the following proven and accurate method for calculating weight can be used.
The first step in calculating weight is to simply identify the springs by inside diameter, wire size and total unwound length. From this information, spring charts can be used to determine the inch pound per turn (IPPT) rate of each spring used in the installation. Your spring, door or drum supplier should make this information available to you.
For example, you have been called by a customer who has an older 14-foot-by-14-foot wood standard-lift door that is not cycling correctly. You would like to update all hardware being used on this installation. The existing springs are both .3625” wire, 34” in unwound length of 6” I.D. By referring to charts in a spring manual, we see that the IPPT rate of each spring is 87.9 for a total of 175.8 IPPT.
Next, we must calculate the amount of torque required to balance the door. It’s necessary at this time to prevent as much friction as possible between the door face and the jamb or stops (i.e. remove stops, adjusting vertical track, etc.). Now wind the springs until the door is balanced so that any effort on the winding bar lifts the door off the floor and lowers it back to the floor (the door is balanced). Once this point is achieved, record the number of turns on each spring. Multiplying the turns by the IPPT results in the MIP (maximum inch pounds). This is the total inch pounds of torque produced on the shaft that balances the door. If one spring is wound nine complete turns, while the other takes 10, multiply as follows:
87.9 IPPT x 9 turns = 791.1 and then 87.9 IPPT x 10 turns = 879.0 MIP Adding the MIP’s together results in 1670.1 MIP.
Finally, the drums must be identified. Technical specifications can be obtained from the drum’s manufacturer or supplier. The most important measurement is the high moment arm, which is the drum’s radius, including cable, where the cable peels off the drum when the door is closed. Actual measurements must be obtained if you can identify the drum or its manufacturer. In cases where you cannot obtain this measurement as accurately as possible, use a set of calipers or similar measurement device. Measure the space between caliper points with one point on the cable at the high moment arm (cable peel-off point) and the other point on the far side of the shaft. One-half of the shaft’s diameter and one-half of the cable diameter must be deducted. With this information, we can calculate the door’s weight.
First, we determined the high moment arm is 2.936”, (OMI 18’ with 3/16” cable per spring charts). If we divide this into the 1670.1 MIP, the result will be the weight being balanced or specifically door’s weight (1670.1 ˆ 2.936 = 569 pounds). With this information, we can calculate new springs if we wish and update the drums to a currently available model. Please note that this procedure also works for both hi-lift and vertical- lift installations.
Tim Kolhoff is the inside and international sales and castings consultant for Arrow Tru-Line Inc., an Archbold, Ohio-based manufacturer and supplier of all related garage door hardware. Mr. Kolhoff invites readers to contact him for related information or to share their tips. To reach him, call 800.446.6433, ext. 316; e-mail firstname.lastname@example.org; visit www.arrowtruline.com.