變(bian)(bian)速(su)(su)箱(xiang)(xiang)是車輛上非常重(zhong)要的(de)(de)部件，它(ta)可以改變(bian)(bian)傳(chuan)(chuan)動(dong)(dong)比，擴大驅動(dong)(dong)輪轉(zhuan)矩(ju)和(he)轉(zhuan)速(su)(su)的(de)(de)作用(yong)(yong)。隨著現(xian)代科(ke)技的(de)(de)發(fa)展，變(bian)(bian)速(su)(su)箱(xiang)(xiang)也有了升級(ji)，從最初的(de)(de)手(shou)動(dong)(dong)變(bian)(bian)速(su)(su)箱(xiang)(xiang)，到(dao)現(xian)在(zai)無級(ji)變(bian)(bian)速(su)(su)箱(xiang)(xiang)，從無同(tong)步(bu)器(qi)到(dao)有同(tong)步(bu)器(qi)，操控越來越方便。目(mu)前工程(cheng)機(ji)(ji)械上廣(guang)泛采(cai)用(yong)(yong)柴(chai)油機(ji)(ji)，其轉(zhuan)矩(ju)與(yu)轉(zhuan)速(su)(su)變(bian)(bian)化范圍小，不(bu)能滿(man)足車輛在(zai)各種工況下對牽引力(li)(li)和(he)行(xing)駛速(su)(su)度的(de)(de)要求，需采(cai)用(yong)(yong)變(bian)(bian)速(su)(su)箱(xiang)(xiang)來解(jie)決這種矛盾。變(bian)(bian)速(su)(su)箱(xiang)(xiang)性(xing)能的(de)(de)優劣是衡量工程(cheng)機(ji)(ji)械動(dong)(dong)力(li)(li)性(xing)、經濟性(xing)及駕駛性(xing)的(de)(de)關鍵。目(mu)前的(de)(de)變(bian)(bian)速(su)(su)系統主要有：機(ji)(ji)械傳(chuan)(chuan)動(dong)(dong)、液(ye)力(li)(li)傳(chuan)(chuan)動(dong)(dong)、靜液(ye)壓傳(chuan)(chuan)動(dong)(dong)。變(bian)(bian)速(su)(su)箱(xiang)(xiang)有人力(li)(li)換擋(dang)和(he)動(dong)(dong)力(li)(li)換擋(dang)，結構有定軸式(shi)和(he)行(xing)星式(shi)。