變(bian)(bian)速(su)(su)箱(xiang)(xiang)是車輛上非常(chang)重要的部件，它可以改變(bian)(bian)傳(chuan)動(dong)比，擴大驅動(dong)輪轉矩和(he)轉速(su)(su)的作用。隨(sui)著現(xian)代科(ke)技(ji)的發展，變(bian)(bian)速(su)(su)箱(xiang)(xiang)也有(you)了(le)升級，從最(zui)初(chu)的手動(dong)變(bian)(bian)速(su)(su)箱(xiang)(xiang)，到(dao)現(xian)在(zai)無級變(bian)(bian)速(su)(su)箱(xiang)(xiang)，從無同(tong)步器(qi)到(dao)有(you)同(tong)步器(qi)，操控(kong)越來越方便。目(mu)前工(gong)程機械(xie)上廣泛(fan)采用柴油(you)機，其轉矩與轉速(su)(su)變(bian)(bian)化范圍(wei)小，不(bu)能(neng)滿足車輛在(zai)各(ge)種工(gong)況下對(dui)牽引(yin)力(li)和(he)行駛速(su)(su)度的要求，需(xu)采用變(bian)(bian)速(su)(su)箱(xiang)(xiang)來解決這種矛盾(dun)。變(bian)(bian)速(su)(su)箱(xiang)(xiang)性(xing)能(neng)的優劣是衡量(liang)工(gong)程機械(xie)動(dong)力(li)性(xing)、經(jing)濟性(xing)及駕駛性(xing)的關鍵。目(mu)前的變(bian)(bian)速(su)(su)系統主要有(you)：機械(xie)傳(chuan)動(dong)、液力(li)傳(chuan)動(dong)、靜液壓傳(chuan)動(dong)。變(bian)(bian)速(su)(su)箱(xiang)(xiang)有(you)人力(li)換(huan)擋和(he)動(dong)力(li)換(huan)擋，結構有(you)定軸式(shi)和(he)行星(xing)式(shi)。