Distributor's Link Magazine Fall 2020 / Vol 43 No 4
38 THE DISTRIBUTOR’S
38 THE DISTRIBUTOR’S LINK Larry Borowski President GREENSLADE & COMPANY INC. 2234 Wenneca Avenue, Fort Worth, TX 76102 TEL 817-870-8888 FAX 817-870-9199 EMAIL sales1@greensladeandcompany.com WEB www.greensladeandcompany.com INSPECTING SCREW AND BOLT STRAIGHTNESS When a screw or bolt’s length exceeds eight times its body diameter (example: 1/2-13 X 4”) straightness becomes a concern for many end users. If a long screw or bolt is not sufficiently straight it can create significant assembly problems for the end user, which can then result in customer complaints or rejects for the fastener supplier. When measuring bolt straightness, any area containing swell under the head should be avoided. ASME B18.5 states; The length of any permissible swell on the body under the head or neck of bolts should be excluded from checks for shank straightness. A good rule of thumb is to hang about 1 times the shank diameter outside of the jaws when performing this check. Straightness Gages in ASME Standards Several of the American Society of Mechanical Engineers (ASME) B18 standards appendices contain an attribute type of gage for determining part straightness. The pictured gage consists of a base plate with one stationary side rail and one adjustable side rail. The gap between the rails is set with a pair of micrometer heads. This style of gage is quite versatile and cost effective, as it will accommodate a range of diameters and lengths. Most of the ASME standards are consistent in their straightness specifications. Screws and bolts up to and including 12 inches in length can be bowed .006 inches per inch of length. Parts exceeding 12 inches can be bowed .008 inches per inch of length. If a user’s application requires a part straighter than these limits, they must specify their required straightness limits on their print and/or purchase order. In the case of our 1/2-13 x 4” long example, the maximum allowable bow would be .024” (4” length x .006” per linear inch). When using this attribute type gage, you would add the allowable bow (.024”) to the basic diameter (.500”) of the bolt, and set the gap in the jaws at .524”. You would then rotate the fastener 360º in between the jaws. If the part binds, you have exceeded your maximum bow or camber. If it rotates freely, you are within your allowable bow or camber. TECHNICAL ARTICLE CONTINUED ON PAGE 118
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