Common Installation Methods
Installation Methods for Metal Rod and Bore Wipers
In this section we examine the primary methods for installing metal wipers in hydraulic and pneumatic cylinders, emphasizing the most common practices and identifying several alternative approaches. We discuss various methods with respect to differences in cylinder configurations, constraints - such as space, cost, materials compatibility, assembly practices, and application requirements, including installation methods for new cylinder designs, as well as integration and retrofit options.
Background
Several factors have a bearing on the selection of installation methods for a rod scraper. Many times, a scraper and wiper are being paired to provide a comprehensive exclusion sealing system to prevent ingression. The interaction and interdependencies between the scraper and wiper play important roles in determining the installation method and housing design. For cylinder manufacturers that produce large quantities of a particular product, an important consideration alongside performance, is the ease of installation and the time involved in installing and verifying (certifying) the scraper on the production line. For an MRO, it may be the ease of retrofitting the scraper that is most important. Constraints such as accessibility to the gland, space to house the seal and the need for post-installation servicing of the seal, define the main requirements to be addressed by the installation method. It should be borne in mind that the means of installing the scraper is usually also the means of servicing and replacing the scraper. In a recent post we emphasized the need to maintain frequent surveillance of the performance of a rod scraper, to identify any impending failure modes. A scraper usually operates in very harsh environments and is exposed to many different potential causes of failure. By identifying such an occurrence quickly, and replacing the scraper, significant damage can be prevented. So installation methods that allow for ease of inspection, cleaning and rapid replacement will accrue significant benefits over time.
Floating Installation
A “floating” scraper is one that relies upon axial and radial clearances to permit freedom of motion within its housing. These clearances permit sufficient movement of the scraper that it can act independent of the head-cap/housing/gland and therefore maintain a close following relationship with the rod. This is a key means of accomplishing the necessary rod contact that is a hallmark of the floating scraper design. Floating scrapers have consistently out-performed alternatives and most modern scraper designs rely upon a floating installation method to achieve the high performance levels necessary to perform the task of excluding contaminants from reciprocating components. But with this, comes the challenge of designing a housing and installation method that is easy and cost effective to implement. Several methods are regularly employed and in the remainder of this article, we will examine several of these and discuss their advantages, disadvantages and implications.
A Historical Viewpoint
Dating back to the 1940’s, the initial Army-Navy (AN) rod wiper standard has evolved over the intervening decades. Laterly known as the MS28776 and MS33675 military standards, these define the geometries of rod scrapers and housings respectively.
Thus, the origins of the means of housing a rod scraper has its roots in the MS33675 standard which was based on a threaded gland installation as depicted to the right. The MS33675 standard specified a recess for rod scraper, defined by three dimensions, the Major Diameter “D” (Housing OD), Depth “J” (Width) and Minor Diameter “E” (Throat). Unlike many other types of seal housings, the seal is not required to pass through the minor diameter during installation, but rather, the Gland is installed (threaded) into the Housing, thus ‘capturing’ or ‘retaining’ the scraper. The term ‘throat’ is often referred to instead, to indicate the clearance of the retaining ring relative to the lip of the scraper, with the relatively small lip of the MS28776 scraper extending outboard inside and hence, clearing this diameter. Threaded glands are still frequently used, as are bolt-on end caps, that incorporate a recess to house the scraper.
Retained Installations
Constant Section Retaining Rings
A common method of retaining a scraper in commercial applications is through the use of a retaining (snap, or constant-section) ring. Typically these are either single or double-wound rings that ‘snap’ into their own groove in the aperture of housing. A separate retaining ring groove is machined into the housing immediately out-board of the main scraper housing groove, with a clearance between the scraper to allow for freedom of movement. Constant section retaining rings are usually selected by referencing the housing diameter and the scraper’s throat clearance. A simple table for recommending possible matches of retaining rings to scrapers is provided at the Gadjets Website.
Replacement - In Service
A significant advantage of the use of a retaining ring is that it can be easily removed from the housing and exposed for cleaning, inspection and even for replacement. It is strongly recommended that all rods, plungers and pistons upon which Hydrowype scrapers operate, be regularly inspected for any conditions that might indicate a failure mode in the rod scraper. Should such conditions arise, it is advisable to immediately replace the scraper. In most installations, this can be done simply, 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.
AC wipers are designed to replace a scraper, in-situ, without the necessity to remove the clevis or rod-end. This generally assumes accessibility to the wiper in the gland, either as held in place by a retaining ring in the gland aperture, or using a standard adapter or even a two-piece modified (Style-ADC) adapter. ADC adapters are similarly fully broken into two halves to permit retrofit onto a cylinder without having to pass the wiper or adapter over the rod end or clevis. Thus a two-piece wiper and adapter can be installed on an in-service cylinder with only a few minutes work and minimal expense. Style-AC Wipers have the same dimensional specifications as standard A-Series (Style-A1,2 &3) wipers and are intended for use with standard AN grooves, keepers and adapters. An installation tool is available to ease installation of the proprietary spring which wraps around the wiper within a retaining groove, and is tensioned into a notch on the wiper face.
It is a practical necessity to consider 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 effectively dislodged, pulverized and eliminated, may cause the scraper to 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 either worn, or under-engineered, and in need of increased 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. As long as intervention occurs 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.
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. While it is designed to improve the operating characteristics of a mechanical system, 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.
Installation Accessories
Keeper/Follower Plate - Installation “Adapters”
A-Series’ and D-Series’ scrapers are offered with a pair of installation adapter accessories, usually constructed of sheet metal, which provide an economical means of attaching a scraper to the front of a gland or housing. This can usually be accomplished using machine-screws, bolts or in the case of tie-rod cylinders,the threaded tie rod ends. The “adapter” is composed of a top (keeper) plate and a bottom (follower) plate that sandwich the scraper, retaining the scraper while providing the necessary clearances to allow the scraper to “float” and therefore, follow the rod, with freedom over the housing/gland.
Use of Adapter or Keeper Plate
To retrofit a wiper onto an existing cylinder without gland modifications, an adapter or custom machined keeper plate can be used. These components situate the wiper outside the gland, with the assembly typically secured to the gland face via bolting.
Installation adapters can be constructed of sheet metal or machined from carbon steel and other materials. They can be painted, plated, and coated in a range of manganese, nitrides and claddings.
Integration with Secondary Wiper
For environments where contaminants range greatly in particle size and form, a secondary wiper may be employed alongside the metal scraper. Modifications like slotted heel scrapers and the installation of a grease fitting/vent portal in the gland wall can help prevent particle buildup and facilitate regular cleaning through grease flushing.
Metal Cages, for Press-In Installation
Each of the A, D and M-Series scraper families are offered in a variety of configurations, including with optional components such as auxiliary wipers and metal installation cages. The metal cage is provided to permit the scraper to be pre-assembled, along with in some cases, a matched wiper or u-cup, so as to create a sub-assembly that can be pressed into an open counterbore. Oftentimes this is highly desirable, on a production line or in automated assembly. Many rod scrapers that are found in older equipment designs that pre-date the 1990’s, used this configuration and at the time, it was commonly known as a “Hydro-lock™” type of installation (alternatively, in some cases as the Garlock™) style installation method. As the choice of rod wiper profiles and materials has greatly proliferated in recent years, it is now less common to see this type of assembly, however, Gadjets still offers a paired wiper with a machined can as standard accessories for all standard A-Series (AC, AW and AWC), D-Series (DC, DW and DWC) and M-Series (MC, MW and MWC) scrapers.
Summing it up: Advantages and Disadvantages
While there is no specific “right way” to install a rod scraper, consideration should be given to the means by which the necessary clearances are afforded in the housing of the scraper. The use of a retaining ring, provides ease of installation, convenience in being able to easily remove the scraper for cleaning and inspection, and even replacement, and positions the wiper at the cylinder's aperture, nearest the source of contaminants. Scraped contaminants can easily migrate away from the site of their removal and are less likely to compact or embed. The two-piece wiper designs (Style-AC) allows for the replacement of a scraper without dismantling adjacent components, such as the clevis or rod-end.
Installation adapters, comprising a keeper and follower plate are available for standard A-Series wipers, including metric options, while keeper plates offer custom solutions tailored to specific cylinder configurations.
Style-AC scrapers feature a two-piece design that allows for installation without cylinder disassembly. These designs incorporate a unique spring and installation cavity, facilitating the use of a specialized tool for spring tensioning and placement. Each installation method offers specific benefits tailored to the operational requirements and design constraints of hydraulic and pneumatic cylinders.
In conclusion, this article covers various installation methods for metal rod and bore wipers in hydraulic and pneumatic cylinders, highlighting primary practices and alternative approaches. It addresses factors such as cylinder configurations, space constraints, materials compatibility, and application requirements, covering installation methods for both new designs and retrofit options. From floating installations to retained methods like constant section retaining rings, the article explores historical perspectives, practical considerations for in-service replacement, integration with secondary wipers, and the use of installation accessories like keeper plates. Overall, it provides insights into optimizing the performance and longevity of metal wipers in demanding industrial settings.