三(san)(san)(san)(san)輥穿孔的(de)橫斷面變(bian)形(xing)(xing)(xing)(xing)(xing)過(guo)程(cheng)是由(you)圓形(xing)(xing)(xing)(xing)(xing)到圓三(san)(san)(san)(san)角(jiao)形(xing)(xing)(xing)(xing)(xing)再(zai)到圓形(xing)(xing)(xing)(xing)(xing)的(de)過(guo)程(cheng)。這種三(san)(san)(san)(san)角(jiao)形(xing)(xing)(xing)(xing)(xing)的(de)變(bian)形(xing)(xing)(xing)(xing)(xing)特(te)征，使毛管尾(wei)部易出(chu)現(xian)三(san)(san)(san)(san)角(jiao)形(xing)(xing)(xing)(xing)(xing)。通常，鋼管外徑與壁厚之(zhi)比D／S大(da)于9時(shi)，會出(chu)現(xian)尾(wei)三(san)(san)(san)(san)角(jiao)形(xing)(xing)(xing)(xing)(xing)，D／S值(zhi)愈大(da)，尾(wei)三(san)(san)(san)(san)角(jiao)形(xing)(xing)(xing)(xing)(xing)也愈大(da)。尾(wei)三(san)(san)(san)(san)角(jiao)的(de)形(xing)(xing)(xing)(xing)(xing)成(cheng)，限制了在三(san)(san)(san)(san)輥穿孔機上(shang)穿制薄壁管，這是其缺點之(zhi)一。

  三輥(gun)穿(chuan)(chuan)(chuan)孔(kong)(kong)機(ji)(ji)軋輥(gun)形成(cheng)(cheng)了(le)(le)一個近似封閉的(de)(de)(de)孔(kong)(kong)型，不(bu)(bu)需(xu)要像在(zai)二(er)輥(gun)穿(chuan)(chuan)(chuan)孔(kong)(kong)機(ji)(ji)上(shang)為保持管(guan)坯在(zai)中心線上(shang)和限(xian)制金屬的(de)(de)(de)橫向流動，必須裝(zhuang)設導(dao)板或導(dao)盤(pan)。因此(ci)采用三輥(gun)穿(chuan)(chuan)(chuan)孔(kong)(kong)機(ji)(ji)穿(chuan)(chuan)(chuan)孔(kong)(kong)就(jiu)消除(chu)了(le)(le)現有(you)二(er)輥(gun)穿(chuan)(chuan)(chuan)孔(kong)(kong)機(ji)(ji)存(cun)在(zai)的(de)(de)(de)缺點。不(bu)(bu)僅消除(chu)了(le)(le)不(bu)(bu)利導(dao)板或導(dao)盤(pan)所造成(cheng)(cheng)的(de)(de)(de)金屬應(ying)力狀態，也消除(chu)了(le)(le)過(guo)大能(neng)量(liang)(liang)消耗的(de)(de)(de)導(dao)板和導(dao)盤(pan)，消除(chu)了(le)(le)管(guan)坯與它們(men)之間(jian)的(de)(de)(de)摩擦。降低了(le)(le)設備能(neng)量(liang)(liang)消耗，同時也消除(chu)了(le)(le)導(dao)板、導(dao)盤(pan)的(de)(de)(de)工具成(cheng)(cheng)本，提高了(le)(le)經濟性。