▁the ▁early ▁ 2 0 th ▁century . ▁First ▁invent ed ▁by ▁How don ▁Tor que ▁Lim it ers ▁in ▁ 1 9 7 8 . ▁Their ▁design ▁pat ent ▁GB 1 5 2 0 4 1 6 ▁( A ) ▁incorpor ated ▁one ▁or ▁more ▁she ar ▁p ins ▁extending ▁between ▁two ▁fixing ▁blocks ▁det ach ably ▁secured ▁one ▁to ▁each ▁of ▁the ▁two ▁rot ary ▁members . ▁How don ▁also ▁had ▁the ▁US ▁Pat ent ▁US 3 9 7 3 4 1 2 ▁( A ). ▁Pat ent ▁GB 1 5 2 0 4 1 6 ▁( A ) ▁ ▁- ▁ ▁T OR QUE - LI MIT ING ▁CO UP L ING S ▁ ▁How don ▁still ▁produce ▁tor que ▁limit ers ▁in ▁the ▁UK ▁today ▁from ▁their ▁factory ▁in ▁Str oud ▁Gl ouc esters hire . ▁ ▁S ynchronous ▁magnetic ▁A ▁synchron ous ▁magnetic ▁tor que ▁lim iter ▁uses ▁permanent ▁magnet s ▁mounted ▁to ▁each ▁sh aft , ▁with ▁an ▁air ▁gap ▁between . ▁They ▁are ▁very ▁fast ▁acting , ▁but ▁may ▁have ▁more ▁back l ash ▁than ▁mechanical ▁types . ▁Because ▁there ▁is ▁no ▁mechanical ▁contact ▁between ▁the ▁two ▁sh aft s , ▁they ▁are ▁also ▁used ▁to ▁transmit ▁tor que ▁through ▁a ▁physical ▁bar rier ▁like ▁a ▁thin ▁pl astic ▁wall . ▁On ▁some ▁models , ▁the ▁tor que ▁limit ▁may ▁be ▁adjust ed ▁by ▁changing ▁the ▁gap ▁between ▁the ▁magnet s . ▁ ▁Ball ▁det ent ▁A ▁ball ▁det ent ▁type ▁lim iter ▁transm its ▁force