三(san)(san)輥(gun)穿孔的橫斷面變形(xing)(xing)(xing)(xing)(xing)過程是(shi)由圓形(xing)(xing)(xing)(xing)(xing)到圓三(san)(san)角(jiao)形(xing)(xing)(xing)(xing)(xing)再到圓形(xing)(xing)(xing)(xing)(xing)的過程。這(zhe)種三(san)(san)角(jiao)形(xing)(xing)(xing)(xing)(xing)的變形(xing)(xing)(xing)(xing)(xing)特(te)征(zheng)，使毛管尾(wei)(wei)部易出(chu)現(xian)三(san)(san)角(jiao)形(xing)(xing)(xing)(xing)(xing)。通常，鋼管外(wai)徑與壁厚(hou)之比(bi)D／S大(da)于9時，會出(chu)現(xian)尾(wei)(wei)三(san)(san)角(jiao)形(xing)(xing)(xing)(xing)(xing)，D／S值愈大(da)，尾(wei)(wei)三(san)(san)角(jiao)形(xing)(xing)(xing)(xing)(xing)也愈大(da)。尾(wei)(wei)三(san)(san)角(jiao)的形(xing)(xing)(xing)(xing)(xing)成(cheng)，限制(zhi)了在三(san)(san)輥(gun)穿孔機(ji)上穿制(zhi)薄壁管，這(zhe)是(shi)其缺點之一。

  三輥(gun)(gun)穿(chuan)孔(kong)(kong)機(ji)軋輥(gun)(gun)形成了(le)(le)一個近似(si)封閉的(de)孔(kong)(kong)型，不(bu)需要像在二(er)輥(gun)(gun)穿(chuan)孔(kong)(kong)機(ji)上為(wei)保(bao)持管(guan)坯(pi)在中心線上和(he)限(xian)制(zhi)金屬的(de)橫向流(liu)動，必須裝設導(dao)(dao)板或導(dao)(dao)盤。因此(ci)采用三輥(gun)(gun)穿(chuan)孔(kong)(kong)機(ji)穿(chuan)孔(kong)(kong)就消(xiao)(xiao)除了(le)(le)現有二(er)輥(gun)(gun)穿(chuan)孔(kong)(kong)機(ji)存(cun)在的(de)缺(que)點。不(bu)僅消(xiao)(xiao)除了(le)(le)不(bu)利導(dao)(dao)板或導(dao)(dao)盤所(suo)造成的(de)金屬應力狀態，也(ye)(ye)消(xiao)(xiao)除了(le)(le)過大能(neng)(neng)量消(xiao)(xiao)耗(hao)的(de)導(dao)(dao)板和(he)導(dao)(dao)盤，消(xiao)(xiao)除了(le)(le)管(guan)坯(pi)與它們之間(jian)的(de)摩擦。降低(di)了(le)(le)設備能(neng)(neng)量消(xiao)(xiao)耗(hao)，同時也(ye)(ye)消(xiao)(xiao)除了(le)(le)導(dao)(dao)板、導(dao)(dao)盤的(de)工具成本，提(ti)高了(le)(le)經濟(ji)性(xing)。