In looking through Fred Colvin: "Planing, Shaping, and Slotting" a Lindsay Publications reprint of the 1943 original I ran across a novel table drive. It was a worm and rack design where a motor driven worm engaged a rack attached to the planer table; rotating the worm against the rack resulted in an axial motion of the table.
Here's the novel part. Instead of the rack being cut to the helix angle of the worm and the worm's axis running parallel to the rack motion, the rack is cut straight across and the worm is angled across the rack following its helix. The angle permits the driving shaft to exit the side of the machine at an angle so the motor tucks neatly behind the column.
Five advantages accrue to this arrangement: a large drive reduction, intrinsic stiffness, compact design, low parts count, low manufactured cost. Disadvantages are: highly sensitive to continuous lubrication, expensive oddball worm, and substantial modification of the machine structure is required.
Thanks to the angled worm, tooth contact progresses across the rack avoiding concentrated wear. The progression of contact distributes wear and permits higher loading; the line of tooth bearing is constantly moving. Planer drives are typically 12 to 18 revolutions of the motor per ft of table travel. Thus large axial forces are produced with a single reduction consisting of only the lead of the worm instead of a train of expensive gears.
Since the worm is inclined to the rack the worm's normal pitch would have to be cut to suit it meaning an oddball worm and compound gearing the machine that cuts it. Since we have a helicoid contacting a rack tooth, tooth loading would be line contact where the axial thrust is taken by the tooth flank more or less divided by the number of worm threads engaged. Thus lubrication under heavy load would be sliding contact in boundary mode. Big potential for scuffing.
Strictly as a mental exercise I've been evolving the drive to suit my Rockford planer. I've been assuming the following preliminary design parameters.
- Direct drive from a 1200 RPM 20 HP motor rated 1.15 service factor
- 2 diametral pitch rack
- 5" pitch diameter worm
- 14" length of worm engagement with the rack
- 12,000 lb of thrust at 80 ft per minute by overloading the motor 20%. (note duty cycle)
- 150 ft per minute max table velocity
Thus about 8 complete threads will be engaged. Dividing thread engagement count by total thrust equals 1500 lb per each thread engaged.
- Rack material: 4140 HT to Rc 35
- Worm material: 8620 carburized 0.030" deep HT to Rc 58 .
- Worm inclined by the the helix angle (about 5.7 degrees).
- Flood lubricated with an EP worm gear oil.
- Max duty cycle would be 50% averaging 15 strokes per minute for up to 20 minutes at a time.
- Annual utilization would be 1000 hours where 400 hours would be at max duty cycle.
The manufactuing data and actual machining of the component parts are routine machine shop technique. The drive design is straight forward. The rack can be purchased to most any gearing class affordable from a dozen sources as can the oddball lead worm if it's not made in the shop. Installation and alignment requires judgement and care but, again, nothing out of the ordinary.
We have line contact not area. So the question I've been pondering as an exercise narrows down to this: how do you calculate drive longevity?
Forgot to add: control over direction and speed of the motor would be via a VFD and PLC logic. Adjustable limit stops used in the tradtional manner would set table stroke.
[This message has been edited by Forrest Addy (edited 02-20-2003).]