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PMI

Version A Version B
Version A Version B

Overview


Single Rope
(#818, 2653)

Front View: Closed
Front View: Closed
 
Open for Rigging
Open for Rigging
 
Top View
Top View
 
Rear View
Rear View

Technical Details

I acquired this chest box from PMI in March, 2005. I acquired two more in 2017 as part of Bob Thrun's collection.

My PMI, single rope is 75 mm. long, 197 mm. wide, 54 mm. high, and weighs 301 g. The back plate has 51 mm. tall vertical slots for attaching the box to a chest strap and 26 mm. wide horizontal slots for attaching shoulder straps.

The PMI box consists of a back plate, rupport post, roller axle, roller, and latching gate assembly. The construction made extensive use of CNC machining. The back plate is milled from 3.5 mm. aluminum plate, possibly 7075, and anodized. Although not obvious, the front face is milled to taper to the edges, where the plate is only 6 mm. thick. The back has two large irregular areas where superfluous metal was removed to a depth of 4.3 mm. There are large (51.8 by 6.8 mm.) slots on each side for attaching one's main chest harness strap, and pairs of smaller (26.0 by 6.8 mm.) slots on the top and bottom for attaching the harness shoulder slings.

The post is milled from aluminum, anodized, and bolted to the back plate with three counter-sunk Allen screws, probably 1/4" (6.4 mm.). An extension on the side of the post provides lateral rigidity. It looks heavy, but the post is undercut on the othe side. There is a level area on the plate where the post attaches, so the overall plate taper does not affect the post's alignment.

The roller is 16.5 mm. wide and features a shallow U-shaped rope centering groove. The roller diameter is 28 mm. at the edges and 23.8 mm. in the center. I did not disassemble the roller to determine the type of bearing, but it feels like it is oilite and not a ball or roller bearing. The roller axle appears to be a 5/15" (8 mm.) round-head Allen cap screw with full-length thread secured against the post with a jam nut. I suspect that the post is threaded; otherwise the axle would probably wobble. The roller can wobble on the axle (very slightly - not signicficant at all) but the axle itself is rigidly attached to the post,

The gate is 6.9 mm. thick, but milled from slightly thicker stock, since there are two raised arcs on the roller side that bear against the roller and separate the amin gate body from the roller by about 0.7 mm. Turning the gate does not turn the roller, so these are not the main source of the roller drag. The gate latch is a piece of 2.9 mm. spring wire bent into a zquared-off U shape. When the gate rotates closed, the wire enters a large slot in the back plate. The slot is tapered to flex the wire until it can engage a small projection, where the wire snaps into place, holding the gate shut. A cutout in the gate face provides access for releasing the wire.

The PMI chest box is printed with "rock exotica" (the manufacturer), "USA," "Patent pending," arrows indicating gate operation, the Reading is Dangerous icon, "Always test gate to confirm it is locked," and "Feb. 2005" on the caver's right. It is printed with "Single CHEST ROLLER," "Do not use a chest roller for promary life support," and the PMI logo with "PMI" on the caver's left. A sticker on the back reads, "WARNING" and "FOR DANGEROUS ACTIVITIES RISK OF INJURY/DEATH CNNOT BE ELIMINATED. DO NOT USE UNLESS YOU HAVE: READ ALL INSTRUCTIONS* RECEIVED SUITABLE TRAINING* CHECKED GEAR BEFORE EACH USE* ACCEPTED TOTAL RESPONSIBILITY FOR YOUR SAFETY AND GEAR SUITABILITY. ALWAYS USE A BACKUP - NEVER TRUST A LIFE TO A SINGLE TOOL."

Comments

Tapering the front plate lightens the plate while retaining full strength - as any engineer knows, a center-loaded beam supported at both ends has most of its stress in the center, and should be thicker there. This is a nice feature that shows attention to detail. The milled areas in back lighten the plate considerably, as does the large undercut in the front post.

The gate assembly is particularly easy to operate with either hand. With the box right side up, I can open the gate with my right hand by using my thumb to depress the safety, or by using a finger of my left hand to do the same. I don't even need to grip the gate: thumb/finger pressure suffices to rotate it. The chest box can be turned upside-down for people who prefer using their other hand for manipulating the gate. The gate locks closed reliably with a distinct audible click.

The rollers turn with light resistance. I would prefer a free-running ball or roller bearing here along with a bushing design that allowed it to rotate more freely; however, the drag is small and probably not to be noticed.

The note, "Do not use a chest roller for primary life support" gets my nomination as one of the "DUH!" message of the year. Exactly how would a chest box support my weight, anyhow? No wonder… and by the way, in SRT, one customarily trusts ones life to a single rope.

This box is excellent. The only real complaint I have against this box is that I prefer double roller boxes - and that complaint is addressed by the double roller version.


Double Rope
(#819, 1708, 2607)

Front View: Closed
Front View: Closed
 
Open for Rigging
Open for Rigging
 
Top View
Top View
 
Rear View
Rear View

Technical Details

I acquired this chest box from PMI in March, 2005, and a second from Cargo Largo in 2011. I acquired a third in 2017 as part of Bob Thrun's collection.

My PMI, double rope is 75 mm. long, 197 mm. wide, 54 mm. high, and weighs 353 g. The back plate has 51 mm. tall vertical slots for attaching the box to a chest strap and 26 mm. wide horizontal slots for attaching shoulder straps.

The double rope version has a redesigned post that supports rollers on each side. One roller is centered on the plate, the other is offset to the climber's right. The back plate has three pairs of 7.2 mm. holes, giving additional chouces for attaching bungie pullies and other peripheral items, if one chooses.

The markings on my double are the same as on the single except for their location and that "Double CHEST ROLLER" replaces "Single CHEST ROLLER."

Comments

Offsetting the roller to one side makes some sense. A Mitchell climber would probably place the main line through the central roller and the long foot sling through the offset roller. People who climb with the long sling on the left foot might want to turn the box upside down. Except for the printing, it is symmetrical. On the other hand, some people might argue that a Mitchel climber has their weight on the long foot sling as often as on the main linem so the two rollers should be symmetrically placed about the center. Personally, I doubt that most people could tell the difference.

This may be the finest double chest roller on the market at the time I write this. I saw no compelling reason to give up using my Alpine Box, but this box is so good that I did. Still, one needs to be careful. Peter Jones sent me the following message in November, 2010:

I have a comment on the Rock Exotica double roller chest box. I owned and loved a Friztke Box for many years until my toxic sweat (salt) from an unwashed harness after a caving trip led to the aluminum plate disintegrating. I replaced this in 2006 with a Rock Exotica double roller box, which I felt was a small step up from the Fritzke, both being extremely well designed and constructed. I used it for two years and was about to make a four day trip in Virgin Cave, a serious vertical cave in the Guads. On examining my gear a week before the start of the trip, I noticed, to my horror, that one of the bolts that holds the right hand roller in place was hand loose, capable of coming undone with only the slightest amount of friction and turning. I called PMI who insisted that they be the ones to examine it and determine what to do with it. I insisted that since it was about to be used within two days that it be repaired immediately by the manufacturer. Fortunately, PMI agreed and I drop shipped it directly to Utah for repairs. They repaired it and sent it directly to Carlsbad, NM where I picked it up on arrival.

The problem is that the bolts that hold the rollers and swing plates in position are glued in place, certainly with a high strength adhesive, but my own experience indicates that it can literally "come unglued." The bolts are set at a specified distance into the central post such that there is no friction on the swing plate. This means that the bolts are not set tight against one another which might add some strength to the tightening. There is no set screw on the backside of the bolts to lock it in place, only the glue. In addition, unlike the Friztke Box, which uses a solid bolt all the way through, there can be an inordinate amount of torque on the individual bolts which are held only by a few turns of the screw. If I had used the Rock Exotica Box in its unscrewed position, unbeknownst to me, it could have torqued its way right out of the central post, leaving me in some seriously compromised position on rope.

I have written the manufacturer about my concerns over the design, but there has never been any response. I assume that it still remains as designed. As such, I always make sure that I fully examine the bolts that hold the rollers in place before embarking on any vertical caving trip. While I still feel that it is about the best designed double roller box on the market, it does have its problems and design flaws.

The adhesive is probably something like Loktite®. Peter still prefers the PMI (Rock Exotica) box, as do I, but his note emphasizes the importance of checking one's gear before relying on it.