{"id":13,"date":"2026-03-26T14:07:00","date_gmt":"2026-03-26T13:07:00","guid":{"rendered":"https:\/\/www.matai.cz\/blog\/trida-pevnosti-8-8\/"},"modified":"2026-06-26T14:57:17","modified_gmt":"2026-06-26T12:57:17","slug":"trida-pevnosti-8-8","status":"publish","type":"post","link":"https:\/\/www.matai.cz\/blog\/trida-pevnosti-8-8\/","title":{"rendered":"T\u0159\u00edda pevnosti 8.8: co znamenaj\u00ed \u010d\u00edsla, Rm a Re a srovn\u00e1n\u00ed s 10.9, 12.9 a nerezem"},"content":{"rendered":"<p>Ozna\u010den\u00ed <strong>8.8<\/strong> vyra\u017een\u00e9 na hlav\u011b \u0161roubu pat\u0159\u00ed mezi nej\u010dast\u011bj\u0161\u00ed &ndash; je to pevnostn\u00ed t\u0159\u00edda uhl\u00edkov\u00e9 (ocelov\u00e9) oceli podle normy ISO 898-1. Co p\u0159esn\u011b \u010d\u00edsla znamenaj\u00ed, jak z nich spo\u010d\u00edtat pevnost a jak se 8.8 li\u0161\u00ed od vy\u0161\u0161\u00edch t\u0159\u00edd 10.9 a 12.9 i od nerezu, vysv\u011btluje tento \u010dl\u00e1nek.<\/p>\n<h2>Co znamenaj\u00ed \u010d\u00edsla 8.8<\/h2>\n<p>Dvoj\u010d\u00edsl\u00ed s te\u010dkou k\u00f3duje mechanick\u00e9 vlastnosti \u0161roubu:<\/p>\n<ul>\n<li><strong>Prvn\u00ed \u010d\u00edslo (8)<\/strong> &times; 100 = minim\u00e1ln\u00ed <strong>pevnost v tahu Rm<\/strong> v MPa &rarr; 8 &times; 100 = <strong>800 MPa<\/strong>.<\/li>\n<li><strong>Druh\u00e9 \u010d\u00edslo (.8)<\/strong> je pom\u011br meze kluzu k mezi pevnosti (0,8). <strong>Mez kluzu Re (Rp0,2)<\/strong> = 800 &times; 0,8 = <strong>640 MPa<\/strong>.<\/li>\n<\/ul>\n<p>\u0160roub t\u0159\u00eddy 8.8 tedy mus\u00ed m\u00edt minim\u00e1ln\u00ed pevnost v tahu 800 MPa a minim\u00e1ln\u00ed mez kluzu 640 MPa. Vyr\u00e1b\u00ed se z <strong>kalen\u00e9 a popou\u0161t\u011bn\u00e9 (zu\u0161lecht\u011bn\u00e9) uhl\u00edkov\u00e9 oceli<\/strong> a je to nejpou\u017e\u00edvan\u011bj\u0161\u00ed vysokopevnostn\u00ed t\u0159\u00edda v b\u011b\u017en\u00e9m stroj\u00edrenstv\u00ed.<\/p>\n<h2>Srovn\u00e1n\u00ed t\u0159\u00edd 8.8, 10.9 a 12.9<\/h2>\n<table border=\"1\" cellpadding=\"6\" cellspacing=\"0\">\n<thead>\n<tr>\n<th>T\u0159\u00edda<\/th>\n<th>Pevnost v tahu Rm (min.)<\/th>\n<th>Mez kluzu Re \/ Rp0,2 (min.)<\/th>\n<th>Materi\u00e1l<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>8.8<\/td>\n<td>800 MPa<\/td>\n<td>640 MPa<\/td>\n<td>zu\u0161lecht\u011bn\u00e1 uhl\u00edkov\u00e1 ocel<\/td>\n<\/tr>\n<tr>\n<td>10.9<\/td>\n<td>1000 MPa<\/td>\n<td>900 MPa<\/td>\n<td>zu\u0161lecht\u011bn\u00e1 legovan\u00e1 ocel<\/td>\n<\/tr>\n<tr>\n<td>12.9<\/td>\n<td>1200 MPa<\/td>\n<td>1080 MPa<\/td>\n<td>zu\u0161lecht\u011bn\u00e1 legovan\u00e1 ocel<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Stejn\u00fd princip v\u00fdpo\u010dtu plat\u00ed pro v\u0161echny t\u0159\u00eddy: u 10.9 je Rm 10 &times; 100 = 1000 MPa a mez kluzu 1000 &times; 0,9 = 900 MPa; u 12.9 je Rm 1200 MPa a mez kluzu 1200 &times; 0,9 = 1080 MPa. Vy\u0161\u0161\u00ed t\u0159\u00eddy nesou vy\u0161\u0161\u00ed zat\u00ed\u017een\u00ed, ale jsou tvrd\u0161\u00ed, k\u0159eh\u010d\u00ed a citliv\u011bj\u0161\u00ed na korozn\u00ed prask\u00e1n\u00ed &ndash; zejm\u00e9na 12.9 vy\u017eaduje opatrnost p\u0159i povrchov\u00e9 \u00faprav\u011b (riziko vod\u00edkov\u00e9 k\u0159ehkosti).<\/p>\n<h2>Srovn\u00e1n\u00ed s nerezem A2-70 a A4-80<\/h2>\n<p>U nerezov\u00e9ho spojovac\u00edho materi\u00e1lu se pou\u017e\u00edv\u00e1 jin\u00e9 zna\u010den\u00ed (dle ISO 3506), ale princip je obdobn\u00fd: \u010d\u00edslo &times; 10 d\u00e1v\u00e1 p\u0159ibli\u017en\u011b desetinu pevnosti v tahu.<\/p>\n<table border=\"1\" cellpadding=\"6\" cellspacing=\"0\">\n<thead>\n<tr>\n<th>Materi\u00e1l \/ t\u0159\u00edda<\/th>\n<th>Pevnost v tahu Rm<\/th>\n<th>Mez kluzu Rp0,2<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Ocel 8.8<\/td>\n<td>800 MPa<\/td>\n<td>640 MPa<\/td>\n<\/tr>\n<tr>\n<td>Nerez A2-70<\/td>\n<td>700 MPa<\/td>\n<td>450 MPa<\/td>\n<\/tr>\n<tr>\n<td>Nerez A4-80<\/td>\n<td>800 MPa<\/td>\n<td>600 MPa<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Z tabulky je vid\u011bt, \u017ee nerez A4-80 se v pevnosti v tahu vyrovn\u00e1 oceli 8.8 (ob\u011b 800 MPa), ale m\u00e1 ni\u017e\u0161\u00ed mez kluzu (600 vs 640 MPa). A2-70 je o stupe\u0148 n\u00ed\u017ee. Pozor: nerez a uhl\u00edkov\u00e1 ocel se li\u0161\u00ed nejen pevnost\u00ed, ale i korozn\u00ed odolnost\u00ed &ndash; nerez nekoroduje, ale za stejnou pevnost \u010dasto zaplat\u00edte v\u00edc.<\/p>\n<p>V praxi tedy vol\u00edte podle kombinace zat\u00ed\u017een\u00ed a prost\u0159ed\u00ed: pro such\u00e9 vnit\u0159n\u00ed zat\u00ed\u017een\u00e9 spoje pozinkovan\u00e1 <a href=\"\/p\/sroub-m-6-x-80-6hr-cz-din-933-a2\/\">ocel t\u0159\u00eddy 8.8<\/a>, pro korozn\u00ed prost\u0159ed\u00ed <a href=\"\/p\/srouby-s-sestihrannou-hlavou-s-drikem-din-931-a4\/\">nerez A4<\/a>, p\u0159\u00edpadn\u011b pro b\u011b\u017en\u00e9 prost\u0159ed\u00ed <a href=\"\/p\/srouby-s-sestihrannou-hlavou-s-drazkou-din-933-a2\/\">nerez A2<\/a>.<\/p>\n<h2>Zna\u010den\u00ed na hlav\u011b \u0161roubu<\/h2>\n<p>Pevnostn\u00ed t\u0159\u00edda je u ocelov\u00fdch \u0161roub\u016f vyra\u017eena p\u0159\u00edmo na hlav\u011b (nap\u0159. &bdquo;8.8&ldquo;, &bdquo;10.9&ldquo;, &bdquo;12.9&ldquo;) spolu se zna\u010dkou v\u00fdrobce. U nerezov\u00fdch \u0161roub\u016f najdete na hlav\u011b materi\u00e1lov\u00e9 a pevnostn\u00ed ozna\u010den\u00ed (nap\u0159. &bdquo;A2-70&ldquo; nebo &bdquo;A4-80&ldquo;). V\u017edy si zna\u010den\u00ed p\u0159ekontrolujte &ndash; je to nejrychlej\u0161\u00ed zp\u016fsob ov\u011b\u0159en\u00ed t\u0159\u00eddy.<\/p>\n<h2>Z \u010deho vych\u00e1z\u00ed syst\u00e9m zna\u010den\u00ed<\/h2>\n<p>Syst\u00e9m pevnostn\u00edch t\u0159\u00edd ocelov\u00fdch \u0161roub\u016f definuje norma <strong>ISO 898-1<\/strong>. Jeho elegance spo\u010d\u00edv\u00e1 v tom, \u017ee z dvoj\u010d\u00edsl\u00ed lze p\u0159\u00edmo &bdquo;p\u0159e\u010d\u00edst&ldquo; ob\u011b kl\u00ed\u010dov\u00e9 hodnoty bez nutnosti hledat v tabulk\u00e1ch. Stejn\u00fd kl\u00ed\u010d funguje nap\u0159\u00ed\u010d celou \u0159adou t\u0159\u00edd (4.6, 4.8, 5.6, 5.8, 8.8, 9.8, 10.9, 12.9): prvn\u00ed \u010d\u00edslo &times; 100 d\u00e1v\u00e1 mez pevnosti v MPa, sou\u010din obou \u010d\u00edsel &times; 10 d\u00e1v\u00e1 p\u0159ibli\u017en\u011b mez kluzu v MPa. Nap\u0159\u00edklad u t\u0159\u00eddy 5.6 je Rm 500 MPa a mez kluzu 5 &times; 6 &times; 10 = 300 MPa. Toto pravidlo je u\u017eite\u010dn\u00e9 m\u00edt v hlav\u011b p\u0159i rychl\u00e9m posouzen\u00ed dokumentace nebo zna\u010den\u00ed na hlav\u011b \u0161roubu.<\/p>\n<h2>Mez pevnosti Rm a mez kluzu Re v praxi<\/h2>\n<p>Co tyto dv\u011b hodnoty znamenaj\u00ed pro mont\u00e1\u017e? <strong>Mez kluzu Re (Rp0,2)<\/strong> je nap\u011bt\u00ed, p\u0159i kter\u00e9m za\u010dne materi\u00e1l trvale (plasticky) t\u00e9ct &ndash; \u0161roub se nevratn\u011b prodlou\u017e\u00ed. Utahovac\u00ed moment se proto vol\u00ed tak, aby p\u0159edp\u011bt\u00ed ve \u0161roubu z\u016fstalo bezpe\u010dn\u011b pod mez\u00ed kluzu (typicky kolem 70-90 % Rp0,2 podle metody utahov\u00e1n\u00ed). <strong>Mez pevnosti Rm<\/strong> je nap\u011bt\u00ed, p\u0159i kter\u00e9m dojde k p\u0159etr\u017een\u00ed. Mezi mez\u00ed kluzu a mez\u00ed pevnosti m\u00e1 materi\u00e1l je\u0161t\u011b ur\u010ditou ta\u017enost, tak\u017ee pevn\u00fd spoj neprask\u00e1 zcela bez varov\u00e1n\u00ed &ndash; to je d\u016fvod, pro\u010d se p\u0159\u00edli\u0161 k\u0159ehk\u00e9 t\u0159\u00eddy (12.9) v n\u011bkter\u00fdch aplikac\u00edch nedoporu\u010duj\u00ed.<\/p>\n<p>Vy\u0161\u0161\u00ed t\u0159\u00edda neznamen\u00e1 automaticky lep\u0161\u00ed volbu. S rostouc\u00ed pevnost\u00ed kles\u00e1 ta\u017enost a roste citlivost na vruby, korozn\u00ed prask\u00e1n\u00ed a vod\u00edkovou k\u0159ehkost. Pro v\u011bt\u0161inu b\u011b\u017en\u00fdch strojn\u00edch spoj\u016f je 8.8 optim\u00e1ln\u00ed kompromis mezi pevnost\u00ed, hou\u017eevnatost\u00ed a cenou.<\/p>\n<h2>Vliv povrchov\u00e9 \u00fapravy a vod\u00edkov\u00e9 k\u0159ehkosti<\/h2>\n<p>U pevnostn\u00edch t\u0159\u00edd 10.9 a zejm\u00e9na 12.9 hroz\u00ed p\u0159i galvanick\u00e9m (elektrolytick\u00e9m) zinkov\u00e1n\u00ed <strong>vod\u00edkov\u00e1 k\u0159ehkost<\/strong> &ndash; vod\u00edk pronikl\u00fd do oceli m\u016f\u017ee zp\u016fsobit n\u00e1hl\u00e9 prask\u00e1n\u00ed zat\u00ed\u017een\u00e9ho \u0161roubu. Proto se tyto d\u00edly mus\u00ed po pokoven\u00ed odvod\u00edkovat (vyp\u00e9ct) v peci, p\u0159\u00edpadn\u011b se vol\u00ed \u017e\u00e1rov\u00fd zinek \u010di povlaky bez tohoto rizika. T\u0159\u00edda 8.8 je v tomto ohledu mnohem m\u00e9n\u011b citliv\u00e1. Toto je dobr\u00e9 zohlednit u\u017e p\u0159i v\u00fdb\u011bru: pokud pot\u0159ebujete pozinkovan\u00fd velmi pevn\u00fd \u0161roub do vlhka, pora\u010fte se o vhodn\u00e9 povrchov\u00e9 \u00faprav\u011b.<\/p>\n<h2>T\u0159\u00eddy pevnosti u matic<\/h2>\n<p>K pevnostn\u00edmu \u0161roubu pat\u0159\u00ed i odpov\u00eddaj\u00edc\u00edm zp\u016fsobem dimenzovan\u00e1 matice. Matice maj\u00ed vlastn\u00ed zna\u010den\u00ed (nap\u0159. 8, 10, 12), kde \u010d\u00edslo zhruba odpov\u00edd\u00e1 pevnostn\u00ed t\u0159\u00edd\u011b \u0161roubu, se kter\u00fdm se m\u00e1 kombinovat &ndash; matice t\u0159\u00eddy 8 se pou\u017e\u00edv\u00e1 k \u0161roubu 8.8, matice 10 k 10.9 atp. Pou\u017eit\u00ed slab\u0161\u00ed matice na pevn\u011bj\u0161\u00ed \u0161roub by mohlo v\u00e9st k vytr\u017een\u00ed (p\u0159eto\u010den\u00ed) z\u00e1vitu matice d\u0159\u00edve, ne\u017e \u0161roub vyu\u017eije svou pevnost.<\/p>\n<h2>Shrnut\u00ed<\/h2>\n<p>U t\u0159\u00eddy 8.8 znamen\u00e1 prvn\u00ed \u010d\u00edslo pevnost v tahu (800 MPa) a druh\u00e9 pom\u011br meze kluzu k pevnosti (0,8 &rarr; 640 MPa). Vy\u0161\u0161\u00ed t\u0159\u00eddy 10.9 a 12.9 nab\u00edzej\u00ed v\u00edc, ale jsou tvrd\u0161\u00ed, k\u0159eh\u010d\u00ed a citliv\u011bj\u0161\u00ed na vod\u00edkovou k\u0159ehkost. Nerez A4-80 pevnostn\u011b dorovn\u00e1v\u00e1 8.8 (800 MPa) a p\u0159id\u00e1v\u00e1 korozn\u00ed odolnost. K pevnostn\u00edmu \u0161roubu v\u017edy volte odpov\u00eddaj\u00edc\u00ed matici.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Co znamen\u00e1 pevnostn\u00ed t\u0159\u00edda 8.8: Rm 800 MPa, mez kluzu 640 MPa. Srovn\u00e1n\u00ed s 10.9 a 12.9 i s nerezem A2-70\/A4-80 a zna\u010den\u00ed na hlav\u011b \u0161roubu.<\/p>\n","protected":false},"author":1,"featured_media":613,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_matai_meta_desc":"Co znamen\u00e1 pevnostn\u00ed t\u0159\u00edda 8.8: Rm 800 MPa, mez kluzu 640 MPa. Srovn\u00e1n\u00ed s 10.9 a 12.9 i s nerezem A2-70\/A4-80 a zna\u010den\u00ed na hlav\u011b \u0161roubu.","_matai_faq":"[{\"q\": \"Co znamen\u00e1 ozna\u010den\u00ed 8.8 na \u0161roubu?\", \"a\": \"Prvn\u00ed \u010d\u00edslo 8 ud\u00e1v\u00e1 minim\u00e1ln\u00ed pevnost v tahu 8 x 100 = 800 MPa. Druh\u00e9 \u010d\u00edslo .8 je pom\u011br meze kluzu k pevnosti (0,8), tak\u017ee mez kluzu = 800 x 0,8 = 640 MPa. Jde o zu\u0161lecht\u011bnou uhl\u00edkovou ocel.\"}, {\"q\": \"Jak\u00fd je rozd\u00edl mezi 8.8, 10.9 a 12.9?\", \"a\": \"8.8 m\u00e1 Rm 800 MPa a mez kluzu 640 MPa; 10.9 m\u00e1 1000 a 900 MPa; 12.9 m\u00e1 1200 a 1080 MPa. Vy\u0161\u0161\u00ed t\u0159\u00eddy unesou v\u00edc, ale jsou tvrd\u0161\u00ed, k\u0159eh\u010d\u00ed a citliv\u011bj\u0161\u00ed na vod\u00edkovou k\u0159ehkost.\"}, {\"q\": \"Je nerez A4-80 stejn\u011b pevn\u00e1 jako ocel 8.8?\", \"a\": \"V pevnosti v tahu ano, ob\u011b maj\u00ed 800 MPa. Nerez A4-80 m\u00e1 ale o n\u011bco ni\u017e\u0161\u00ed mez kluzu (600 vs 640 MPa) a hlavn\u011b p\u0159id\u00e1v\u00e1 korozn\u00ed odolnost, kterou uhl\u00edkov\u00e1 ocel nem\u00e1.\"}, {\"q\": \"Z \u010deho se vyr\u00e1b\u00ed \u0161rouby t\u0159\u00eddy 8.8?\", \"a\": \"Z kalen\u00e9 a popou\u0161t\u011bn\u00e9 (zu\u0161lecht\u011bn\u00e9) st\u0159edn\u011b uhl\u00edkov\u00e9 oceli. Je to nejpou\u017e\u00edvan\u011bj\u0161\u00ed vysokopevnostn\u00ed t\u0159\u00edda pro b\u011b\u017en\u00e9 stroj\u00edrenstv\u00ed.\"}, {\"q\": \"Kde najdu pevnostn\u00ed t\u0159\u00eddu na \u0161roubu?\", \"a\": \"U ocelov\u00fdch \u0161roub\u016f je vyra\u017eena p\u0159\u00edmo na hlav\u011b (nap\u0159. 8.8, 10.9, 12.9) spolu se zna\u010dkou v\u00fdrobce. U nerezu b\u00fdv\u00e1 na hlav\u011b ozna\u010den\u00ed jako A2-70 nebo A4-80.\"}]","_matai_schema_type":"TechArticle","footnotes":""},"categories":[2],"tags":[],"class_list":["post-13","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-normy-a-materialy"],"_links":{"self":[{"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/posts\/13","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/comments?post=13"}],"version-history":[{"count":1,"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/posts\/13\/revisions"}],"predecessor-version":[{"id":216,"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/posts\/13\/revisions\/216"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/media\/613"}],"wp:attachment":[{"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/media?parent=13"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/categories?post=13"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.matai.cz\/blog\/wp-json\/wp\/v2\/tags?post=13"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}