Mechanical Performance Requirements

SEMS images


Machine Screws with Free-Spinning Lockwashers, SEMS
Mechanical Performance Requirements

Description A machine screw and free-spinning lock washer manufactured as one assembly. Steel SEMS are available with the following types of washers: internal tooth, external tooth, split-lock and square-cone°. Stainless SEMS are offered with external tooth lock washers.
Applications
and
Advantages		 The washer/screw assembly makes this a locking screw with the washer providing the locking action. Machine pre-assembly provides cost savings to the end user.
Internal tooth SEMS: Recommended when it is desirable to hide the teeth for appearance or to prevent snagging.
External Tooth SEMS: Preferred over the internal tooth style as the teeth provide greater torsional resistance being on the larger radius.
Split-Lock SEMS: Preferred over tooth lockwasher SEMS for use with hardened bearing surfaces.
Square-Cone° SEMS: Provides a higher retained clamp load and improved compensation for thermal cycling and vibration. Can accept a high tension load and maintain spring action. The washer design makes for better control during installation, and improved tool bit life. Ideal for clamping fragile materials and for spanning large clearance holes.
Material Steel Screws:   AISI 1022 or equivalent steel
Steel Washers:   Split-lock:  SAE 1055 - 1065
Stainless Screws:   SAE 18-8 stainless steel, passivated
Stainless Washers:   Tooth-lock:  SAE 410 stainless steel
Hardness Steel Screws:   Rockwell B70 - B100
Steel Washers:   Split-lock:  Rockwell C38 - 46   Tooth-lock:  Rockwell C40 - 50   Square-Cone°:  Rockwell C42 - 46
Stainless Screws:   SAE 18-8 stainless steel, passivated
Stainless Washers:   Tooth-lock:  Rockwell C40 - 50
Tensile
Strength		 Steel:  60,000 psi minimum. No. 2 diameter SEMS screws are not subject to tensile testing. No. 4 SEMS screws shorter than 1/2 inch are not subject to tensile testing. SEMS screws of diameters No. 6 to 10 inclusive, which are shorter than 1/2 inch or #D (where D is the nominal screw size in inches) are not subject to tensile testing. Such SEMS screws of a size to be tested shall meet the tensile load requirements given on the thread dimensions page.
Stainless:  No. 4: 99,000 psi min.   No. 6 and No. 8: 96,000psi min.   
Note: Nos. 4, 6 and 8 SEMS screws which are shorter than
1/2 inch are not subject to tensile testing.
Plating See Machine screw plating page for info on the plating of steel SEMS.


How SEMS differ from Machine Screw Specifications


  • The maximum diameter of the unthreaded shank shall be less than the maximum major diameter of the thread by an amount sufficient to prevent disassembly of the washer from the screw.
  • The unthreaded length on the fully threaded screws is measured to the contacting face of the washer instead of the screw bearing face.
  • The minimum underhead fillet radius is equivalent to 5% of the basic screw diameter.

Thread Gaging Notes, Machine Screw Thread Dimensions,
and Mechanical & Performance W/O Washers


Smith Fastener Company • 3613 East Florence Avenue, Bell, California, 90201
Phone: ( 323 ) 587-0382 • FAX: ( 323 ) 587-8712 • E-mail: sales@smithfast.com
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