變(bian)(bian)速箱(xiang)(xiang)(xiang)是車輛(liang)上非常重(zhong)要(yao)的(de)部件，它可以改變(bian)(bian)傳(chuan)動(dong)(dong)比，擴大(da)驅動(dong)(dong)輪轉矩(ju)和轉速的(de)作用。隨著現代科技的(de)發展(zhan)，變(bian)(bian)速箱(xiang)(xiang)(xiang)也有(you)了升(sheng)級(ji)，從最初的(de)手動(dong)(dong)變(bian)(bian)速箱(xiang)(xiang)(xiang)，到現在(zai)無級(ji)變(bian)(bian)速箱(xiang)(xiang)(xiang)，從無同(tong)步器到有(you)同(tong)步器，操控越來越方便。目前工(gong)程機械(xie)上廣(guang)泛采用柴油機，其轉矩(ju)與轉速變(bian)(bian)化(hua)范圍小，不能滿足車輛(liang)在(zai)各種(zhong)工(gong)況下對牽引力(li)和行駛速度的(de)要(yao)求，需采用變(bian)(bian)速箱(xiang)(xiang)(xiang)來解決這種(zhong)矛盾。變(bian)(bian)速箱(xiang)(xiang)(xiang)性(xing)(xing)能的(de)優劣(lie)是衡量工(gong)程機械(xie)動(dong)(dong)力(li)性(xing)(xing)、經濟性(xing)(xing)及駕駛性(xing)(xing)的(de)關(guan)鍵。目前的(de)變(bian)(bian)速系統主要(yao)有(you)：機械(xie)傳(chuan)動(dong)(dong)、液(ye)(ye)力(li)傳(chuan)動(dong)(dong)、靜液(ye)(ye)壓傳(chuan)動(dong)(dong)。變(bian)(bian)速箱(xiang)(xiang)(xiang)有(you)人力(li)換擋和動(dong)(dong)力(li)換擋，結構有(you)定(ding)軸式和行星式。