Service and Performance 

Scraper Wear and Life span

An exclusion seal such as a rod or bore scraper, has a service life that can be defined by several factors:

1. The extent of wear of the bore of the scraper on the rod. This can be measured by a change in the diameter and also by a change in the width of the gap in the circumference of the scraper body (at the point of the slot). A scrapers lifespan can be considered the amount of time it takes for the bore to wear to the extent that the gap in the circumference entirely closes. While the scraper will still act as a physical barrier to help contain ingression, the scraping performance of the seal will be diminished beyond this point.

2. The extent to which the bore of the scraper remains free of damage such as burrs. Even in the event of a significant burr being incurred in the face of the scraper such that material is deflected into the path of the rod, this still should not cause scratching from the scraper. However it might lead to a condition described in 3 below.

3. The extent to which the scraper may encounter foreign material that could become trapped behind or embedded in the body of the scraper, possibly at the face or in the bore. It is almost inevitable that some amount of contaminant bypass will occur over time. Large particles that form a very strong adhesive bond with the surface of a rod or bore may, for a few cycles cause a series of 'collisions' with the face of the scraper and until they are effective pulverized and eliminated, may cause the scraper to 'skip' (rapidly expand and contract) as it passes over the contaminant. This should not occur frequently, as such a condition would suggest that the scraper is under-engineered, and needs increased pre-load (radial loading) a different geometry, or an alternative material, all of which can be configured into a custom Hydrowype scraper. Should bypassed material accumulate, this can embed into the scraper and be dragged by the scraper over the surface of the rod. This condition should become apparent very quickly and an observant operator will maintain an active watch for such an occurrence. The equipment should be stopped immediately and the scraper replaced, per below. If such a condition occurs, as long as intervention is made quickly, the machining action of the replacement scraper over the surface of the rod should be capable of removing, by burnishing, any surface damage that has occurred.

We advise that all rods, plungers and pistons upon which Hydrowype scrapers are operating, be regularly inspected for any of the above conditions. Should these conditions arise, we recommend immediate replacement of the scraper. In most installations, this can be done by removing the retaining ring, backing the scraper out of the housing (usually by extending the rod), opening up the gap in the scraper body to expose the serpentine spring and cutting the spring with wire-cutters, then replacing the scraper assembly with a two-piece (two hemisphere) AC-equivalent. A  description of this part can be found here. This can usually be done in-situ, without removing the clevis/rod end.

It's important to keep in mind that Hydrowype scrapers are used in some of the most harsh conditions, oftentimes on equipment that is in a degraded condition. Furthermore, a rod or bore scraper is a relatively simple device, designed to improve the operating characteristics of a mechanical system. But it's not a panacea. Operating conditions play a very large role in the performance and lifespan of a scraper. Fundamentally, a scraper is no more than a metal ring with certain features that are readily understood and appreciated by mechanical engineers' and incorporated into their designs accordingly. 

Frequency of Replacement

The frequency with which a rod scraper should be replaced can depend on many factors, but anecdotally, based on a large body of  customer feedback, some of the main considerations appear to be:

1. The degree and rate of contamination - the rate at which contaminant particles deposit onto the surface of the rod.

2. The harshness of the contaminants - which can be defined by factors in the nature of the contaminants such as abrasiveness, cohesiveness, and invasiveness, as defined in this context, thus:

   • Abrasiveness - the ability of a contaminant material to scratch and induce wear, in one or more components within the system. 

   • Cohesiveness - the strength of the adhesive bonds that attract and bind contaminant particles to one another, such that contaminant particles act as a body 

   • Invasiveness - the propensity for particles to migrate throughout a mechanical system. This can result from the ability of the particles to form strong adhesive bonds with a mechanical surface, such as the surface of a rod, piston or bore. This can also refer to the "profile" of particles sizes, wherein very fine particles may achieve ingression initially, establishing pathways for consequential ingression of increasingly large particles and where, in general terms, the larger the particle, the greater the potential to accumulate and cause systemic damage.

3. The condition of the rod - indicated by the finish and the indications of wear. Generally speaking, a low finish, especially anything below ST16/ST32, Ra16/RMS16) will result in accelerated wear.

4. Stroke, cycle-rate, operating and ambient temperature and sources of directional heat and cold.

5. Gland/end-cap design/geometry. End-caps that avoid the positioning of the scraper within a recess or counterbore, and thus avoiding the build-up and compaction of contaminants immediately outboard of the scraper, will generally allow for contaminants to evacuate the region being scraped. Conversely, cylinders with scrapers located inside a recess or cylindrical void, can allow for contaminants to accumulate, form strong adhesive bonds, and progressively force their way through the seal.

Considering all of the above and many other factors, the following are anecdotal examples based on our customers feedback from several applications, which indicate the relative frequency of scraper replacement. These examples are not intended for anything other than general informational reference and are not intended as recommendations for any specific application:

Heavily contaminated environment: Cylinder operating bauxite processing facility (pre-aluminum processing application) - presence of large volume of bauxite dust/granular material. D-Series scrapers, replaced on a monthly PM cycle.

Moderately contaminated environment: Cylinder in construction testing application - clean rod of test probe, from soil, sand, gravel, road  and construction dust of various types: A-series scrapers are replaced approximately every 6-12 months.

Lightly contaminated environment: piston in cryogenic pumps, removing ice accumulating from atmospheric humidity on exterior of hydrolox and metholox pump surfaces, A-series scrapers, replaced annually.

It is important to bear in mind that while these replacement rates might seem high, the use of scrapers and their frequent replacement is considered a matter of necessity as despite multiple investigations into alternative cylinder and pump designs, the use of alternate means of preventing ingression or upgrading overall system designs and component specifications to be impervious to the effects of ingression, have proven comparatively costly or undesirable for other reasons. Our goal with hydrowype rod scrapers, is to provide a cost-effective means of controlling ingression. Regular inspection, preventative maintenance and replacement of rods scrapers are an essential component of a Hydrowype solution.

Gadjets make no claims regarding the performance of our scrapers other than that these products are produced according to verifiable manufacturing processes, with care and attention, and should reach our customers in a condition suitable for use.

Scraper Performance - Design Considerations

Mil spec  Manufacturing Standards

The Hydrowype A-Series product line is manufactured to conform to the MS28776 standard, a mil spec that governs aspects of the design such as materials, plating and geometry. As such, each Job Lot of parts are subject to various conformance tests that are intended to prevent defects. A-Series scrapers have a main body that interfaces with the rod, and which are constructed out of one of two copper alloys - UNS36000 or UNS93200. 

We anticipate that a standard chrome rod that the scraper will be installed upon, will have an approximate hardness of Rockwell C 65-68 (150Kg) - at 900 HV (Vickers), with substrate hardness of approximately 45-50 HRC (450-520 HV). A direct comparison of the hardness of the scrapers is not particularly straightforward but to try to give a sense for the difference, neither of the copper alloys we use to manufacture the bodies have a hardness that is typically measured on the Rockwell C Scale 150Kg. UNS36000 as machined, has a nominal hardness of approximately 72 on the Rockwell B Scale 100 Kg and UNS93200 has a Brinell hardness of approx. 65. To try to get to a basis for comparison, 100 on the Rockwell B Scale roughly equates to about 22 on the Rockwell C Scale which is about 240 Brinell. So, naively, we could say that the hard chrome is approximately 3 times harder than the material that the scraper body is constructed of. These parts are plated with Cadmium/Gold Chromate which is called out in the mil spec. This is intended to improve lubricity. Cad plating is also very soft, and acts as a further safeguard against scratching.

Most materials used to produce Hydrowype scraper bodies are selected for their resistance to work-hardening. This ensures that their characteristics will not change as a result of the motion of the scraper body over the rod, over time.  This also depends upon special processing, fabricating and machining techniques that ensure work-hardening is not introduced during production. To this end Gadjets uses several techniques throughout our manufacturing processes. Raw stock is normalized and annealed before and after forming and machining. The order of operations used in producing machined seal bodies is designed to avoid any change in material properties. Specifically, the final two operations performed on scraper bodies are the face groove, which leaves a margin for the precision finish machining operation, the bore taper. On most materials (all copper alloys) these operations are performed with a relatively small depth of cut and with very light machining forces so as to avoid any change in properties. Conversely, materials that are prone to work-hardening are cut with maximum (within tolerance) depth of cut at high feed rates, requiring very stable machining setups and powerful equipment, which assures minimum, controlled changes.

Scratch Resistance (of Rod)

A-Series rod scrapers are tested using an approximation of a Mohs scratch test. Mohs is a comparative test used to determine whether one material can scratch another. Chromium, (hard chrome plating is an electroplating process in which chromium is deposited from a chromic acid solution) at between 8.5 and 9 on the Mohs hardness scale, is considered the hardest metal and the third hardest element after carbon and boron. The Mohs scale is a relative system in which the hardness of a sample is determined by its ability to scratch other samples of known hardness. By comparison, the two copper alloys we use in constructing the rod scrapers have a Mohs hardness of 3.0 (and for further comparison, Iron has 4.0, Steel, 4.0, and Cobalt, 5.0). So in short, aside from various precious metals, the copper alloys are some of the softest metals. Theoretically, by the definition of the Mohs scale of mineral hardness, materials with a higher rating cannot be "visibly" scratched by materials with a lower rating (not accounting for the energy deployed), although it is viewed that if materials are close in Mohs values, microscopic dislocations on the harder material can emerge by vigorously trying to scratch it with the softer material.

Conformance Tests

All rod scrapers produced by Gadjets, are subjected to the conformance tests required for mil spec rod scrapers, namely, those defined in the MIL-R-5049 procurement specification.

Gad-Jets maintains the following capabilities in support of our ability to undertake rod scraper conformance testing:

1. Testing facilities to permit the performance of internal strain of copper-base alloys.

2. A complete set of MIL-G-5514 conformant high and low plug gauges (shown in below photograph)

3. Tapered mandrels to permit conducting tests for springiness (Also shown in pictures, above)

4. A wear test setup (cycling setup), such as is illustrated in page 11 of MIL-R-5049D (right) (figure 1 and photograph of wear test setup)

5. Jigs to retain a wiper ring in conformance with the low tolerance requirements of MS33675, and which hold the rod in a vertical position. 

6. Light sources, adapters and viewers to permit the performance of tests

7. Equipment to support the Pre-aging of type R, class 2, wiper rings

8. Jigs adequate for the purpose of performing Breakout friction tests on all dash sizes. 

9. Other facilities sufficient to meet the requirements for Packaging and Preservation of parts, Marking and other requirements so to meet all qualification and conformance requirements set forth in MIL-R-5049D.

MIL-R-5049 Qualification Testing

Gad-Jets maintains the following in support of our ability to perform customer qualification testing per QPL-5049:

1. Testing facilities to permit the performance of internal strain of copper-base alloys, per 4.5.1 of MIL-R-5049.

2. A complete set of MIL-G-5514 conformant high and low plug gauges as described in 4.5.2. (shown in below photograph)

3. Tapered mandrels to permit conducting tests for springiness per 4.5.6. (Also shown in pictures, above)

4. A wear test setup (cycling setup), such as is illustrated in page 11 of MIL-R-5049D (right), and referenced in section 4.5.5 (figure 1 and photograph of wear test setup)

5. Jigs to retain a wiper ring in conformance with the low tolerance requirements of MS33675, and which hold the rod in a vertical position. 

6. Light sources, adapters and viewers to permit the performance of tests such as described in 4.5.2.1

7. Equipment to support the Pre-aging (4.5.3.1) of type R, class 2, wiper rings

8. Equipment to support testing in accordance with 4.5.3.2. and 4.6.3.3. For Type R and Type M, class 2 wiper rings. 

9. Jigs adequate for the purpose of performing the Breakout friction (4.5.6) test on all dash sizes. 

10. Other facilities sufficient to meet the requirements for Packaging and Preservation of parts, Marking and other requirements so to meet all qualification and conformance requirements set forth in MIL-R-5049D.

On request, we can provide any of the following:

1. Example MIL-STD-831 format test reports, geometry and workmanship inspection  reports describing current manufacturing practices and conformance of Gad-Jets GI-1171 wiper rings to MS28776 Rev D.

2. A Batch Qualification Test report (2018) describing the qualification requirements met as described in MIL-R-5049 - notably including rod contact, de-icing, breakout friction, wear, springiness, identification of product, and workmanship.

Gad-Jets, Inc., is owned by Martin Fluid Power, an American company, privately owned by US citizens, operating in a related field with knowledge and expertise that will be employed to protect and preserve the ability to produce parts conforming to MS28776, I/A/W MIL-R-5049D.