變(bian)(bian)速(su)(su)(su)箱(xiang)(xiang)(xiang)(xiang)是(shi)車輛上非常重要(yao)的部件(jian)，它可以(yi)改變(bian)(bian)傳(chuan)動(dong)(dong)(dong)(dong)比(bi)，擴大(da)驅動(dong)(dong)(dong)(dong)輪轉矩(ju)和(he)轉速(su)(su)(su)的作用。隨著(zhu)現代科技的發(fa)展(zhan)，變(bian)(bian)速(su)(su)(su)箱(xiang)(xiang)(xiang)(xiang)也(ye)有(you)(you)了升級(ji)，從最初的手(shou)動(dong)(dong)(dong)(dong)變(bian)(bian)速(su)(su)(su)箱(xiang)(xiang)(xiang)(xiang)，到現在(zai)無(wu)級(ji)變(bian)(bian)速(su)(su)(su)箱(xiang)(xiang)(xiang)(xiang)，從無(wu)同步器到有(you)(you)同步器，操控越(yue)來(lai)越(yue)方便(bian)。目前工(gong)程機(ji)(ji)械上廣泛采用柴油機(ji)(ji)，其轉矩(ju)與轉速(su)(su)(su)變(bian)(bian)化范(fan)圍小，不能滿足車輛在(zai)各種(zhong)工(gong)況下對牽(qian)引(yin)力(li)和(he)行(xing)駛速(su)(su)(su)度的要(yao)求，需(xu)采用變(bian)(bian)速(su)(su)(su)箱(xiang)(xiang)(xiang)(xiang)來(lai)解決這種(zhong)矛盾。變(bian)(bian)速(su)(su)(su)箱(xiang)(xiang)(xiang)(xiang)性(xing)(xing)能的優劣是(shi)衡(heng)量工(gong)程機(ji)(ji)械動(dong)(dong)(dong)(dong)力(li)性(xing)(xing)、經濟性(xing)(xing)及駕駛性(xing)(xing)的關鍵。目前的變(bian)(bian)速(su)(su)(su)系(xi)統主(zhu)要(yao)有(you)(you)：機(ji)(ji)械傳(chuan)動(dong)(dong)(dong)(dong)、液力(li)傳(chuan)動(dong)(dong)(dong)(dong)、靜液壓傳(chuan)動(dong)(dong)(dong)(dong)。變(bian)(bian)速(su)(su)(su)箱(xiang)(xiang)(xiang)(xiang)有(you)(you)人力(li)換擋和(he)動(dong)(dong)(dong)(dong)力(li)換擋，結(jie)構有(you)(you)定軸式和(he)行(xing)星式。