Optimas Parts Imperial Blog Series: Technical Insight Into Hardened and Unhardened Flat Washers

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Optimas designed the Parts Imperial Blog Series to provide detailed information about the fasteners you use and the ones you may need. Curated to help you find the right parts, these technical releases supply information on components’ specifications, applications and standards. This Imperial Series is comprised of five types of components: flat washers, hex cap screws, locknuts, machine screws and tapping screws. Once you have determined the parts suited for your application, Optimas can source them through manufacturing and distribution, ensuring you get the parts you need.

Hardened Flat Washers

Hardened flat washers are designed for assembly with a Grade 5 or higher strength bolt between the bearing surface of the fastener and the part to which it is attached. They are made of carbon steel and may either be thru-hardened to Rc 38-45 or carburized (case hardened) to a minimum depth of .015” and have a surface hardness of 37-45 Rc with a minimum core hardness of Rc 30.

Unhardened Flat Washers

Unhardened flat washers are designed for assembly with Grade 2 or lower strength bolts or tapping screws between the bearing surface of the fastener and the part to which it is attached. Narrow (SAE) and wide (USS) flat washers are available for specification—depending on the preferred outside diameter needed.

Hardened and Unhardened Flat Washers Are Used To:

  • Improve stress distribution on the mating surface
  • Span larger clearance holes
  • Protect the mating surface from damage during assembly
  • Provide a consistent bearing surface between the mating surfaces for torquing

Dimensional characteristics are covered by ANSI B18.22.1* and mechanical properties are covered by ASTM F844.

Top and side views of a hardened flat washer

Top and side views of a hardened or unhardened flat washer with the dimensional attributes listed in the charts below.

Hardened Flat Washers Specification Chart

NOMINAL SIZE**
(BOLT SIZE)		 A
INSIDE DIAMETER		 B
OUTSIDE DIAMTER		 C
THICKNESS
NOM MIN MAX NOM MIN MAX MIN MAX
1/4 (.250) 9/32 .281 .312 5/8 .594 .656 .051 .080
5/16 (.312) 11/32 .344 .375 11/16 .657 .719 .051 .080
3/8 (.375) 13/32 .406 .437 13/16 .781 .843 .051 .080
7/16 (.438) 15/32 .469 .500 59/64 .891 .953 .051 .080
1/2 (.500) 17/32 .531 .562 1-1/16 1.032 1.094 .097 .177
5/8 (.625) 11/16 .688 .719 1-5/16 1.281 1.343 .122 .177
3/4 (.750) 13/16 .812 .843 1-15/32 1.438 1.500 .122 .177
7/8 (.875) 15/16 .937 .968 1-3/4 1.719 1.781 .136 .177
1 (1.000) 1-1/8 1.125 1.156 2 1.969 2.031 .136 .177

*Sizes indicated in this chart are generally available.
**These are common dimensions in inches.

Unhardened Flat Washers Specification Chart

NOMINAL SIZE**
(BOLT SIZE)		 SERIES A
INSIDE DIAMETER		 B
OUTSIDE DIAMTER		 C
THICKNESS
NOM MIN MAX NOM MIN MAX NOM MIN MAX
#10 (.190) Narrow 7/32 .214 .227 1/2 .495 .515 3/64 .036 .065
Wide 7/32 .214 .227 1/2 .495 .515 3/64 .036 .065
#12 (.216) Narrow 1/4 .245 .265 9/16 .557 .577 1/16 .051 .080
Wide 1/4 .245 .265 9/16 .557 .577 1/16 .051 .080
1/4 (.250) Narrow 9/32 .276 .296 5/8 .620 .640 1/16 .051 .080
Wide 5/16 .307 .327 3/4 .727 .749 1/16 .051 .080
5/16 (.312) Narrow 11/32 .339 .359 11/16 .681 .703 1/16 .051 .080
Wide 3/8 .370 .390 7/8 .868 .905 5/64 .064 .104
3/8 (.375) Narrow 13/32 .401 .419 13/16 .805 .827 1/16 .051 .080
Wide 7/16 .433 .453 1 .993 1.030 5/64 .064 .104
7/16 (.438) Narrow 15/32 .464 .484 59/64 .915 .937 1/16 .051 .080
Wide 1/2 .495 .515 1-1/4 1.243 1.280 5/64 .064 .104
1/2 (.500) Narrow 17/32 .526 .546 1-1/16 1.055 1.092 3/32 .074 .121
Wide 9/16 .557 .577 1-3/8 1.368 1.405 7/64 .086 .132
5/8 (.625) Narrow 21/32 .649 .686 1-5/16 1.305 1.342 3/32 .074 .121
Wide 11/16 .681 .718 1-3/4 1.743 1.780 9/64 .108 .160
3/4 (.750) Narrow 13/16 .805 .842 1-1/2 1.462 1.499 9/64 .108 .160
Wide 13/16 .805 .842 2 1.993 2.030 5/32 .122 .177
7/8 (.875) Narrow 15/16 .931 .968 1-3/4 1.743 1.780 9/64 .108 .160
Wide 15/16 .931 .968 2-1/4 2.243 2.280 11/64 .136 .192
1 (1.00) Narrow 1-1/16 1.055 1.092 2 1.993 2.030 9/64 .108 .160
Wide 1-1/16 1.055 1.092 2-1/2 2.493 2.530 11/64 .136 .192

*Common dimensions of unhardened steel washers are given in inches.

Get a Technical Consult

For more information on hardened and unhardened flat washers or any type of fastener, contact one of our experts at optimas.com/contact.

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