What comes first in case accessories selection, weight or model?

Answer: Weight. It sets the handle and wheel rating; the model sets mounting position.

How do you estimate cycles with no records?

Answer: Daily actions times 250 working days, rounded up: 8 for stores, 32 for changeover, 100 in the field.

Is there a standard spares ratio for case accessories?

Answer: No single figure. Units times rate times lead time: 30 latches and 45 wheels per 1,000 cases on a quarterly cycle.

Does consolidation reduce reliability?

Answer: Not if it merges upward. A heavier part only raises reliability; the risk is a heavy case on a light part.

Where does a sample of 13 come from?

Answer: A 500-unit lot at level II. Double the lot and the sample goes to 20.

Case Accessories Selection: Load, Cycle Count and Spares

Quick answer: Case accessories selection runs on packed weight, cycles a year and batch size: weight sets handle and wheel ratings, cycles set hinge and latch grades, batch size sets spares.

Three Inputs Behind the Choice
Ratings from Packed Weight
Grades from Opening Cycles
Replacement Rate and Spares
Consolidation, Sampling, Rejection
FAQ

1. Three Inputs Behind the Choice

Case accessories are specified from three numbers: packed weight, opening cycles a year, batch size. Fill those in and the handle and wheel ratings, the hinge and latch grades and the spares quantity fall into a range that only needs rounding. Start from a catalogue instead and you get a 120 kg handle on a 12 kg case.

Weight is measured with the case full, not from shell weight. Cycles are daily openings times 250 working days. Batch is the first order plus three years of replenishment.

2. Ratings from Packed Weight

Handle rating runs at 2.2 times packed weight: 12 kg takes 25 kg, 28 kg takes 60 kg, 55 kg takes 120 kg. The factor covers one-handed carry, where one grip more than doubles the moment at the bosses.

Wheels are rated per wheel. A four-wheel case rolls on three points most of the time, so each carries 0.45 of the total: 5.4 kg at 12 kg, 12.6 at 28, 24.8 at 55. Catalogue ratings are static; in motion about 60 percent applies, so a 55 kg case needs 40 kg wheels.

Packed weight	Handle	Per wheel
12 kg	25 kg	5.4 kg
28 kg	60 kg	12.6 kg
55 kg	120 kg	24.8 kg

Ratings follow the weight class, not the case model: one wheel part for two classes either drags the light case or ovals under the heavy one.

3. Grades from Opening Cycles

Cycles come in three bands: stores at 8 a day, 2,000 a year; line changeover at 32 a day, 8,000 a year; field service at 100 a day, 25,000 a year, all on 250 working days. No dimension reveals cycles, yet they drive life harder than size.

Hinge grade is read from axial play: a POM bush holds growth under 0.2 mm to 8,000 cycles, and at 25,000 the pin goes from 4 to 6 mm with a grease port. The latch bottleneck is the hook mouth, which opens up past 20,000 cycles and drops preload by 30 percent.

Higher grades cost more, so specify by station: one case can carry a 2,000-cycle latch in stores and a 25,000-cycle latch in field.

4. Replacement Rate and Spares

Case accessories replacement rate is failures in a year over units in service. Without history, start at 5 percent for handles, 12 percent for latches, 18 percent for wheels and 4 percent for hinges. Wheels run highest: tread wear follows the floor.

Case accessories spares equal units times rate times lead time. For 1,000 cases, latches at 120 a year on a one-quarter lead give 30 in stock; wheels at 180 give 45; handles at 50 give 13.

Part	Annual rate	Per 1,000 units	Quarter stock
Handles	5%	50	13
Latches	12%	120	30
Wheels	18%	180	45
Insert clips	25%	250	63

A year of cover means plastic ageing on the shelf: a nylon latch loses 20 percent of its toughness in two years.

5. Consolidation, Sampling, Rejection

Consolidating case accessories starts with merging part numbers: a project ran nine models on nine sets, and merging handles into two boss pitches, wheels into two ratings and latches into two pulls left four sets with 78 percent common parts.

Merge case accessories on failure consequence: load-bearing parts upward only, since a light case on a heavier set costs less than stocking another part number, and sealing parts downward while preload holds.

A 500-unit lot at inspection level II draws 13 units, with zero critical defects, AQL 1.0 major and 2.5 minor, checking seating, action and cracks. Write the limits down or one inspector passes what another rejects.

Check	Method	Accept	Reject
Boss crack	Visual, 5× lens	None	3 mm long
Tread wear	Outside diameter	Under 1 mm	1.5 mm
Latch preload	Feeler on mark	Loss under 25%	Loss at 40%
Hinge play	Dial gauge, side push	Under 0.3 mm	0.5 mm

Conclusion

Case accessories selection reduces to arithmetic: weight times 2.2 for the handle, 0.45 for each wheel, units times rate times lead time for spares. Cycles fix the grades, batch size fixes how far parts merge. Put the formulas on the purchase order and case accessories stop being a matter of experience.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.