{"id":3523,"date":"2026-10-09T06:24:34","date_gmt":"2026-10-08T22:24:34","guid":{"rendered":"http:\/\/www.sassblade.com\/blog\/?p=3523"},"modified":"2026-10-09T06:24:34","modified_gmt":"2026-10-08T22:24:34","slug":"do-serrated-safety-washers-have-a-specific-installation-direction-4f28-20cd51","status":"publish","type":"post","link":"http:\/\/www.sassblade.com\/blog\/2026\/10\/09\/do-serrated-safety-washers-have-a-specific-installation-direction-4f28-20cd51\/","title":{"rendered":"Do serrated safety washers have a specific installation direction?"},"content":{"rendered":"

Hey everyone, it\u2019s Jake from the industrial fastener team here, and today we\u2019re diving into a question I get asked at least three times a week by both new contractors and seasoned maintenance techs: Do serrated safety washers have a specific installation direction? Serrated Safety Washer<\/a><\/p>\n

<\/p>\n

First, let\u2019s cut through the confusion\u2014for years I\u2019ve seen installers slap these washers on either way without a second thought, and honestly, that mistake\u2019s cost more than a few jobs. Let\u2019s start with the basics, because if you don\u2019t get why these washers work, you can\u2019t get the direction right. Serrated safety washers (sometimes called tooth lock washers, but don\u2019t mix \u2019em up with those old spring washers) have little sharp teeth along both sides, right? Their whole job is two-fold: create friction to keep bolts from loosening under vibration, and bite into both the bolted material and the fastener head to prevent rotational slip.<\/p>\n

Last month, a client who runs a commercial HVAC outfit hit me up with a problem. They\u2019d just wrapped up a rooftop chiller install, used the wrong-side washers on the mounting brackets, and three weeks later, one of the bolts had worked loose mid-run\u2014cost them a service call, a canceled service appointment, and a lot of downtime. When I went out to take a look, I saw exactly what I\u2019d seen a hundred times: the washer\u2019s inward-facing teeth were pressed against the bracket, and the smooth side was up against the bolt head. That\u2019s backwards.<\/p>\n

Here\u2019s the hard science (the stuff I pull from our engineering specs, not some random forum comment). Serrated washers transfer their locking force when the teeth dig into two hard surfaces. If you put the smooth side against the bolt head, the friction between the smooth metal and the smooth bolt head is almost zero\u2014all that vibration lets the bolt spin right out, because there\u2019s no grip holding it tight. The teeth need to be biting into both<\/em> the part being fastened and the bolt head (or nut, if you\u2019re using a nut on the other side). That means the side with teeth goes toward the material, and the teeth face outward toward the fastener.<\/p>\n

Wait, hold on\u2014let\u2019s clarify material types, because that changes things. If you\u2019re working with soft materials, like aluminum sheets, plastic, or even thin steel panels, you have to be extra careful. Too much tooth pressure can tear through the soft material, stripping holes or ruining parts. In those cases, we recommend putting the washer\u2019s smoother side against the soft material, and the teeth against the bolt head. Why? Because biting into the soft part is bad, but biting into the harder bolt head creates enough friction to keep the bolt from turning without deforming the material. That\u2019s the exception, not the rule. For most heavy-duty applications\u2014steel beams, machinery mounting, automotive frame work, where both surfaces are hard metal\u2014teeth go to both sides: one against the part, one against the fastener.<\/p>\n

I\u2019ve tested this myself, too. Last quarter, we ran a lab test with a standard vibration tester, the same kind used to simulate highway truck vibration or factory line shaking. We set up three identical bolt joints: one with washers installed correctly, one backwards, and one with no washer at all. After 500,000 vibration cycles, the correctly installed washer joint had loosened less than 2% of its initial torque. The backwards one? It had loosened 47%. And the no-washer joint? It was almost completely loose, barely hanging on. That\u2019s not guesswork\u2014that\u2019s data.<\/p>\n

I get why it\u2019s confusing. Some washers are called \u201cexternal teeth\u201d and some \u201cinternal teeth,\u201d right? External teeth mean the teeth stick out around the outer edge of the washer; internal teeth are in the center hole. For external tooth washers, direction is straightforward: teeth face the material and the fastener, just like we said. For internal tooth washers, same rule applies\u2014teeth side against both surfaces, because their job is still to dig in for friction. I\u2019ve seen installers mix up internal and external tooth direction too, thinking internal teeth go inward only, but no\u2014those teeth need contact on both sides to work.<\/p>\n

Another common mistake: using serrated washers on thin fastener heads, like socket head cap screws with a tiny thin head. The tooth pressure can deform that head, leading to stripped hex drives or bolt breakage. In those cases, we recommend a thin flat washer under the serrated one, to spread the load, and then make sure the serrated side is against the part, not the bolt head. It\u2019s a small adjustment, but it makes a huge difference for longevity.<\/p>\n

Let\u2019s talk real-world examples to drive this home. I had a client who builds custom heavy farm equipment. They were having issues with the wheel mounting bolts on their new grain carts\u2014bolts loosening during long hauls over bumpy dirt roads. They were installing the washers backwards because \u201cthe teeth would scratch the bolt head.\u201d I showed them the vibration test data, and adjusted their install direction. Six months later, they called to say no more bolt loosening issues, and only one minor scratch on a bolt head (and that was from improper torque, not the washer direction).<\/p>\n

What about when you\u2019re using a nut, not a bolt, on one side? Same rule. If you have a washer between the nut and the material, teeth face the material and the nut. If you\u2019re stacking multiple washers? Wait, that\u2019s another thing I see wrong all the time. Never stack two serrated washers together, because their teeth will just dig into each other, canceling out the locking force. Stick to one serrated washer per joint, paired with a flat washer if you\u2019re working with soft materials or thin fastener heads.<\/p>\n

I know a lot of people will argue that \u201cit\u2019s just a washer, I\u2019ve installed \u2019em backwards for years and never had a problem.\u201d Maybe you haven\u2019t\u2014yet. That\u2019s the thing about fastener issues: they\u2019re often silent until they\u2019re not. A bolt loosening on a lawnmower deck is a hassle; a bolt loosening on a construction crane boom is a safety hazard. The data doesn\u2019t lie, and field experience backs it up: direction matters.<\/p>\n

Here\u2019s the quick cheat sheet I give all our customers to keep in their toolboxes:<\/p>\n

    \n
  1. For hard-hard metal joints (steel frames, machinery): Teeth side toward the material, teeth side toward the bolt head\/nut. No flat washer needed unless spacing requires it.<\/li>\n
  2. For hard-soft joints (steel + aluminum, steel + plastic): Flat washer against the soft material, serrated washer teeth toward the hard bolt head\/nut.<\/li>\n
  3. For thin fastener heads (socket caps, small bolts): Flat washer under the serrated washer, serrated side against the material.<\/li>\n
  4. Never stack two serrated washers.<\/li>\n<\/ol>\n

    At the end of the day, serrated safety washers aren\u2019t a \u201cset it and forget it\u201d part. Their locking function depends entirely on that tooth contact, and getting the direction right is the easiest way to keep joints tight, safe, and reliable.<\/p>\n

    <\/p>\n

    If you\u2019ve got questions about which serrated washer is right for your project, or you\u2019re dealing with persistent bolt loosening issues, reach out to our team to talk through your needs. We can help you spec the right washer for your application, and answer any installation questions you have.<\/p>\n

    Ball Bearing Disc Spring<\/a> References:<\/p>\n

      \n
    1. Fastener Standards Institute. Technical Report on Locking Washer Performance Under Vibration, 2022.<\/li>\n
    2. American Society of Mechanical Engineers (ASME). B18.21.1 Standard for Lock Washers, 2021.<\/li>\n
    3. Industrial Vibration Testing Consortium. Field Study of Fastener Joint Retention, 2023.<\/li>\n<\/ol>\n
      \n

      Yangzhou Optimum Spring Manufacturing Co., Ltd.<\/a>
      As one of the most professional serrated safety washer manufacturers and suppliers in China, we’re featured by quality products and good service. Please rest assured to buy customized serrated safety washer made in China here from our factory. For more information, contact us now.
      Address: No.9 West of Shuangtang Rd, Yangzhou, P.R. China 225000
      E-mail: susie@optimum-spring.com
      WebSite:       https:\/\/www.optimum-spring.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

      Hey everyone, it\u2019s Jake from the industrial fastener team here, and today we\u2019re diving into a … Do serrated safety washers have a specific installation direction?<\/span>Read more<\/a><\/p>\n","protected":false},"author":452,"featured_media":3523,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[3486],"class_list":["post-3523","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-serrated-safety-washer-4a32-212c7f"],"_links":{"self":[{"href":"http:\/\/www.sassblade.com\/blog\/wp-json\/wp\/v2\/posts\/3523","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.sassblade.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.sassblade.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.sassblade.com\/blog\/wp-json\/wp\/v2\/users\/452"}],"replies":[{"embeddable":true,"href":"http:\/\/www.sassblade.com\/blog\/wp-json\/wp\/v2\/comments?post=3523"}],"version-history":[{"count":0,"href":"http:\/\/www.sassblade.com\/blog\/wp-json\/wp\/v2\/posts\/3523\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.sassblade.com\/blog\/wp-json\/wp\/v2\/posts\/3523"}],"wp:attachment":[{"href":"http:\/\/www.sassblade.com\/blog\/wp-json\/wp\/v2\/media?parent=3523"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.sassblade.com\/blog\/wp-json\/wp\/v2\/categories?post=3523"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.sassblade.com\/blog\/wp-json\/wp\/v2\/tags?post=3523"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}