JP2002122212A - Sealing structure of differential device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sealing structure surely preventing lubricating oil in a housing from flowing out from an axle insertion hole to the outside with the axle detached from an axle insertion hole of side gears by internally fitting plugs in fitting hole portions of the respective side gears of a differential device. SOLUTION: The fitting hole portions 20 recessed outward in the direction of an axis a and communicated with the axle insertion hole 6c are formed in an axial inside end portion in the housing in the respective side gears of the differential device, the recessed-cross-sectional plugs 30 recessed inward in the direction of the axis a of the side gears 6 are provided, and the plugs 30 are internally fitted in the fitting hole portions 20 to seal clearances between the plugs 30 and the side gears 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a seal structure for a differential gear, and more particularly, to a fitting hole recessed outward in the axial direction at an axially inner end portion of each side gear. Further, the present invention relates to a structure in which a plug having a concave cross-section recessed inward in the axial direction is internally fitted to seal between the plug and the side gear.

[0002]

2. Description of the Related Art Conventionally, as shown in FIGS. 6 and 7, for example, a differential device 100 for a vehicle basically includes:
Ring gear 101, gear case 102, a pair of pinion gears
103, a pair of side gears 104, these members 101 to 104
And a housing 105 for accommodating the same. A pair of axles
106 is inserted through the opening 105a of the housing 105 into the interior thereof, and is inserted through the gear case 102 to form a pair of side gears 104.
Are inserted and spline-coupled to the through-shaft axle insertion holes 104a. The driving force input to the ring gear 101 is transmitted to a pair of axles 106 (wheels) via a gear case 102, a pinion gear 103, and a pair of side gears 104.

[0003] Lubricating oil is contained in the housing 105, and the lubricating oil lubricates and cools the driving parts such as the gears 101, 103, 104 and the sliding parts thereof. The lubricating oil is supplied from the opening 105a of the housing 105. In order not to leak outside
As shown in FIG. 6, when the axle 106 is inserted into the axle insertion hole 104a, the axle 106 and the housing 105 are inserted through the opening 105a.
Is sealed by the seal member 107.

[0004]

However, as shown in FIG. 7, when the axle 106 is removed from the axle insertion hole 104a of the side gear 104 by inspection, repair, or replacement of the axle 106 or parts related thereto, the housing 105 The lubricating oil inside may flow out from the opening 105a, and if so,
After the axle 106 is inserted into the axle insertion hole 104a, the housing
The 105 needs to be refilled (filled) with lubricating oil, which is very disadvantageous because of unnecessary work and cost.

Accordingly, the inventor of the present application has set the outer end of the side gear in the axial direction so that the lubricating oil in the housing does not flow out from the opening even when the axle is removed from the axle insertion hole of the side gear. At the opening, the side gear and the housing are sealed with a seal member at the opening, and a general plug is previously fitted in the axle insertion hole at the axially inner end portion of the side gear. In addition, an attempt was made to apply a structure for sealing between the plug and the side gear.

However, when a general plug is fitted in the axle insertion hole of the side gear so as to be internally fitted, the length of the axle insertion portion inserted into the axle insertion hole becomes shorter than that of the conventional one.
That the existing axle is difficult to assemble, that the connection strength between the axle and the side gears may be reduced, that it is difficult to easily and securely attach the plug to the axle insertion hole,
It was concluded that there were problems such as difficulty in improving the sealing performance for sealing between the plug and the side gear.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a seal structure for a differential device in which a plug is fitted in a fitting hole of each side gear, and lubricating oil in a housing is inserted into the axle when the axle is removed from the axle insertion hole of the side gear. Reliably preventing leakage from the hole to the outside, securing the desired connection strength between the axle and the side gear by allowing the existing axle to be assembled, and enhancing the sealing performance for sealing between the plug and the side gear; And that the plug can be easily and reliably mounted in the fitting hole.

[0008]

According to a first aspect of the present invention, a seal structure for a differential device includes a ring gear, a gear case, a pair of pinion gears, a pair of side gears, and a housing for accommodating these members. In a differential device for a vehicle in which an axle is inserted and releasably connected to a penetrating axle insertion hole of each side gear whose outer end portion facing the outside of the housing faces the outside of the housing to transmit driving force, a shaft in the housing of each side gear is provided. At the inner end portion in the axial direction, a fitting hole is formed that is recessed outward in the axial direction and communicates with the axle insertion hole, and a plug having a concave cross-section recessed inward in the axial direction of the side gear is provided. Fits inside the fitting hole to seal between the plug and the side gear, preventing leakage of lubricating oil inside the housing from the axle insertion hole of the side gear It is characterized by having done.

A plug is mounted in advance in a fitting hole of each side gear, and in this state, a pair of axles are respectively inserted and connected to axle insertion holes of the pair of side gears. The peripheral wall of the plug having a concave cross section is elastically deformed to be firmly fitted into the fitting hole and closely attached to the plug, thereby improving the sealing performance for sealing between the plug and the side gear. Therefore, even when the axle is removed from the axle insertion hole of the side gear, the lubricating oil in the housing can be reliably prevented from flowing out of the axle insertion hole.

[0010] Since the plug has a concave cross-section recessed inward in the axial direction of the side gear, the axle can be inserted inside the plug even if the plug is fitted inside the fitting hole. . Therefore, it is not necessary to shorten the length of the axle insertion portion to be inserted into the axle insertion hole, so that the existing axle can be assembled, and the desired connection strength between the axle and the side gear can be achieved. Can be secured.
It is preferable that the axially outer end portion of each side gear is protruded to the outside from the opening of the housing, and a seal member seals between the side gear and the housing at the opening.

According to a second aspect of the present invention, in the seal structure for a differential device according to the first aspect of the present invention,
A taper inner surface portion whose diameter becomes smaller toward the inner side in the axial center direction, and a stepped portion extending from the end of the taper inner surface portion on the side of the axle insertion hole toward the axis and having a larger diameter than the axle insertion hole; And a tapered peripheral wall portion elastically deformed and fitted inside the tapered inner surface portion in a state of being substantially in contact with the step portion.

With the end of the peripheral wall portion of the plug almost in contact with the step of the fitting hole, the peripheral wall portion is elastically deformed and is firmly fitted into the tapered inner surface of the fitting hole portion and closely adhered thereto. . Moreover,
Since the inner diameter of the tapered inner surface of the fitting hole becomes smaller toward the inner side in the axial direction, the plug fitted by fitting the peripheral wall to the inner surface of the tapered inner surface can be surely prevented. In this manner, the sealing performance for sealing between the side gear and the axle can be reliably improved, and the plug can be easily and reliably mounted in the fitting hole.

According to a third aspect of the present invention, in the seal structure of the differential device, the peripheral wall portion is elastically deformed when the plug is mounted on a portion axially inside the tapered inner surface portion of the fitting hole portion. And a taper guide portion for guiding so as to perform the operation. By pushing the plug into the fitting hole, first, the peripheral wall of the plug is guided by the tapered guide portion of the fitting hole so as to be elastically deformed. The formed peripheral wall portion can be fitted inside the tapered inner surface portion. Thus, the plug can be easily mounted simply by pushing the plug into the fitting hole.

According to a fourth aspect of the present invention, in the seal structure for a differential device according to the third aspect of the present invention, the taper angle of the tapered inner surface of the fitting hole is larger than the taper angle of the peripheral wall of the plug. It is a feature. It is possible to further enhance the sealing performance for sealing between the plug and the side gear while improving the plug retaining effect.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. The present embodiment is an example of a case where the present invention is applied to a differential device of an automobile (commonly known as FF vehicle) that mounts an engine on a front portion of a vehicle body and transmits a driving force to left and right front wheels, and the differential apparatus is , And the transmission.

As shown in FIGS. 1 and 2, the differential device 1 accommodates a ring gear 2, a gear case 3, a pair of pinion gears 4, a pinion shaft 5, a pair of left and right side gears 6, and these members 2 to 6. A housing 7 and the like are provided. The housing 7 is formed integrally with a transmission case 9, and a transmission (not shown) is provided in the case 9, and a driving force is transmitted from the transmission to the differential device 1.

The ring gear 2 is fixed to the gear case 3 so as to be fitted externally, and the gear case 3 is rotatably supported by the housing 7 via a pair of bearings 10. Although not shown, the ring gear 2 is meshed with a drive gear fixed to a drive shaft of the transmission, and the ring gear 2 is driven to rotate integrally with the gear case 3 by a driving force input from the drive gear.

As shown in FIGS. 1 to 3, a gear housing 3a is formed in a central portion of the gear case 3, and a pair of a pinion gear 4 and pinion shafts 5 and 1 are formed in the gear housing 3a.
The gear portions 6a of the pair of side gears 6 are housed. The pinion shaft 5 is the rotation axis of the gear case 3 (the direction of the axis a).
And both ends thereof are inserted into the gear case 3, at least one end of which is inserted into the pin 11.
Through the gear case 3. The pair of pinion gears 4 are disposed to face each other with the rotation shaft of the gear case 3 interposed therebetween, and are rotatably fitted to the pinion shafts 5.

The pair of side gears 6 have a gear portion 6a and a sleeve 6b, respectively, and are disposed so as to be opposed to each other with the pinion shaft 5 interposed therebetween and the direction of the axis a coincides with the rotation axis of the gear case 3. A pair of side gears 6 on the pinion gear 4
Gear portion 6a is meshed. A large part of the sleeve 6b of each side gear 6 is fitted in the gear case 3 and is rotatably supported. The outer end of the sleeve 6b in the direction of the axis a protrudes from the opening 7a of the housing 7 to the outside. The opening 7a is sealed by a seal member 12 between the side gear 6 and the housing 7.

Each of the pair of side gears 6 has an axle insertion hole 6c penetrating therethrough in the direction of the axis a, and the outer end of the side gear 6 in the direction of the axis a faces the outside of the housing 7, as shown in FIG. The housing 7 is inserted into the axle insertion hole 6c of the side gear 6.
The base end portions of the pair of axles 15 are respectively inserted from the outside and spline-coupled so as to be releasable. The driving force input to the ring gear 2 is the gear case 3 and the pinion shafts 5 and 1.
The power is transmitted to a pair of axles 15 via a pair of pinion gears 4 and a pair of side gears 6, and the left and right wheels attached to these axles 15 are rotated to drive the vehicle.

When the vehicle travels straight, the driving force input to the ring gear 2 causes the pair of pinion gears 4 to
It rotates (revolves) integrally with the other, but does not rotate, but transmits the same driving force to the pair of side gears 6 to drive the left and right wheels. When traveling on a curve, since the resistance from the road surface acting on the left and right wheels is different, the driving force of the outer wheel is automatically increased with respect to the driving force of the inner wheel according to the difference in the resistance.
The pair of pinion gears 4 rotate while rotating integrally with the gear case 3, transmit different driving forces to the pair of side gears 6, differentially drive the left and right wheels, and travel smoothly on a curve.

Now, the housing 7 contains lubricating oil, and the lubricating oil lubricates and cools the driving parts such as the gears 2, 4, 6 and the sliding parts thereof. As shown in the figure, the lubricating oil in the accommodating portion 3a of the gear case 3 is prevented from flowing out of the housing 7 through the axle insertion hole 6c of the side gear 6 when the axle 15 is detached from the side gear 6. This is a specific differential device seal structure.

In this sealing structure, as shown in FIGS. 1 to 3, each of the side gears 6 is recessed outward in the direction of the axis a at the inner end of the side gear 6 in the direction of the axis a within the housing 7 and has an axle insertion hole 6c. A plug 30 having a concave cross-section recessed inward in the direction of the axis a of the side gear 6 is provided, and the plug 30 is internally fitted into the fitting hole 20 to form the communicating hole 20. 6 are sealed.

As shown in FIGS. 4 and 5, the fitting hole 20
Is a tapered inner surface portion 21 whose diameter decreases toward the inner side in the axis a direction,
The end of the tapered inner surface portion 21 on the side of the axle insertion hole 6c (outside in the direction of the axis a) protrudes toward the axis a and forms the axle insertion hole 6c.
A step portion 22 having a larger diameter than the tapered inner surface portion 21 and first and second tapered guide portions 23 and 24 formed at a position inside the taper inner surface 21 in the direction of the axis a and having a larger diameter toward the inside in the direction of the shaft center a.

As shown in FIG. 4, the minimum diameter of the inner end of the tapered inner surface portion 21 in the axial center a direction is substantially the same as the minimum diameter of the outer end of the second tapered guide portion 24 in the axial center a direction. The maximum diameter of the inner end in the direction of the axis a of the 2 taper guide portion 24 is substantially the same as the minimum diameter of the outer end of the first taper guide portion 23 in the direction of the axis a, and the taper inner surface portion 21 and the second taper guide portion. 24, the second taper guide 24 and the first taper guide 23
Between them, inner surface portions 25 and 26 are respectively formed.

As shown in FIGS. 4 and 5, the plug 30
A tapered peripheral wall portion 31 that is elastically deformed and internally fitted to the tapered inner surface portion 21 in a state of being in contact with the step portion 22 of the fitting hole portion 20.
And a lid 32 that closes the inside of the peripheral wall 31 in the axis a direction and swells inward in the direction of the axis a. The peripheral wall 31 and the lid 32 are integrally formed. The peripheral wall portion 31 is formed into a taper whose diameter increases toward the outside in the axial center a direction. In a state shown in FIG. 4 in which the plug 30 is removed from the fitting hole 20, the maximum diameter of the outer peripheral end of the peripheral wall portion 31 in the axial center a direction is The first of the fitting holes 20
The diameter of the tapered guide portion 23 is smaller than the maximum diameter at the inner end in the direction of the axis a.

The first and second taper guides 23, 24
When the plug 30 is mounted in the fitting hole 20, the peripheral wall 3
The first tapered guide portion 23 is a portion for guiding the first tapered guide portion 23 to be elastically deformed. As shown by an arrow in FIG. As a result, the peripheral wall portion 31 is elastically deformed in the diameter reducing direction. Further, when the plug 30 is pushed into the fitting hole portion 20, the peripheral wall portion 31 is further elastically deformed in the diameter reducing direction by the second tapered guide portion 24, The end of the peripheral wall portion 31 substantially abuts the step 22, and the elastically deformed peripheral wall portion 31 forms the tapered inner surface portion 2.
1 inside.

Here, the tapered inner surface 21 of the fitting hole 20
Is larger than the taper angle of the peripheral wall portion 31 of the plug 30. As shown in FIG. 1, a clip 16 which is a C-shaped retaining ring is provided near a base end of each axle 15.
The clip 16 prevents the axle 15 from being removed from the side gear 6 in a state where the axle 15 is inserted into the axle insertion hole 6c.

The operation and effect of the seal structure of the differential device 1 will be described. As described above, the plug 30 is mounted in the fitting hole 20 of each side gear 6 in advance, and in this state, a pair of axles are inserted into the axle insertion holes 6c of the pair of side gears 6 as shown in FIG. 15 are inserted and spline-connected. Before the pair of axles 15 are inserted into the axle insertion holes 6c, the housing 7 is filled with lubricating oil.

The peripheral wall portion 31 of the plug 30 having a concave cross section is elastically deformed and firmly fitted into and tightly attached to the tapered inner surface portion 21 of the fitting hole portion 20, thereby providing a sealing property for sealing between the plug 30 and the side gear 6. Can be increased. Therefore, FIG.
The axle 15 is inserted into the axle insertion hole 6 of the side gear 6 as shown in FIG.
Even when the lubricating oil is removed from the housing 7, the lubricating oil in the housing 7 can be reliably prevented from flowing out of the axle insertion hole 6c to the outside.

Therefore, there is no fear that the lubricating oil in the housing 7 will flow out of the axle insertion hole 6c to the outside due to inspection, repair, replacement, or the like of the axle 15 or parts related thereto, and the axle 15 is connected to the axle insertion hole of the side gear 6. 6c, it is not necessary to refill (fill) the housing 7 with lubricating oil after the axle 15 is again inserted into the axle insertion hole 6c, so that unnecessary work and cost are not required. It will be advantageous. Further, even if the axle 15 is not inserted into the axle insertion hole 6c, the lubricating oil in the housing 7 does not flow out to the outside. Therefore, when assembling the differential device 1 (transmission) in an automobile production line or the like, the housing 7 (casing) It is possible to perform the operation without the lubricating oil flowing out in a state where the lubricating oil is filled therein.

Since the plug 30 has a concave cross section that is recessed inward in the direction of the axis a of the side gear 6, even if the plug 30 is fitted in the fitting hole 20 as shown in FIG. The axle 15 can be inserted inside the plug 30. Therefore, it is not necessary to shorten the length of the axle insertion portion inserted into the axle insertion hole 6c as compared with the conventional one.
The existing axle 15 can be assembled, and the desired connection strength between the axle 15 and the side gear 6 can be secured.

The fitting hole 20 is formed between the tapered inner surface 21 and the step 2
Since the plug 30 has the peripheral wall 31, the peripheral wall 31 is elastically deformed in a state where the end of the peripheral wall 31 of the plug 30 is almost in contact with the step 22, and is firmly attached to the tapered inner surface 21. It is fitted inside and closely adhered to it. Moreover, the tapered inner surface portion 21
Since the diameter becomes smaller toward the inner side in the axial center a direction, the peripheral wall portion 31 is fitted inside the tapered inner surface portion 21 so that the plug 30 to be mounted can be reliably prevented from coming off. That is, the sealing performance for sealing between the side gear 6 and the axle 15 is surely enhanced,
The plug 30 can be easily and securely mounted in the fitting hole 20.

A first and a second taper guide portion 23 for guiding the peripheral wall portion 31 to be elastically deformed when the plug 30 is mounted at a position inside the tapered inner surface portion 21 of the fitting hole portion 20 in the direction of the axis a. , 24, the plug 30 is pushed into the fitting hole 20, so that the peripheral wall 31 is first guided by the first and second taper guides 23, 24 so that the peripheral wall 31 is elastically deformed. Push step 22
, And the elastically deformed peripheral wall portion 31 can be fitted inside the tapered inner surface portion 21.

As described above, the plug 30 can be easily mounted simply by pushing the plug 30 into the fitting hole 20. Since the taper angle of the tapered inner surface portion 21 of the fitting hole portion 20 is larger than the taper angle of the peripheral wall portion 31 of the plug 30, the effect of preventing the plug 30 from coming off is improved.
It is possible to further enhance the sealing performance for sealing between the gear and the side gear 6.

Instead of directly projecting the side gear 6 from the opening 7a of the housing 7 to the outside, the gear case 3
(A cylindrical portion supporting the side gear 6) is projected outside through an opening 7a of the housing 7, and the housing 7 and the gear case 3 are sealed with a sealing member at the opening 7a. Is also good. If there is a possibility that the lubricating oil may flow out from between the gear case 3 and the side gear 6, it is necessary to seal the gap therebetween.

In the above-described embodiment, the present invention is applied to a differential apparatus for a vehicle (commonly referred to as an FF vehicle) in which an engine is mounted on a front portion of a vehicle body to drive front wheels. (Commonly referred to as FR vehicle) or a vehicle (commonly referred to as FR) that drives the rear wheels by mounting an engine in the center or rear of the vehicle body
The present invention can also be applied to a differential device mounted on a car or a midship car). In addition, various modifications other than the above-described modification can be made without departing from the spirit of the present invention.

[0038]

According to the seal structure of the differential device of the first aspect, a fitting hole that is recessed outward in the axial direction at the axially inner end portion in the housing of each side gear and communicates with the axle insertion hole. A plug having a concave section was formed on the inner side of the side gear in the axial direction. The peripheral wall of the plug having a concave cross section is elastically deformed and firmly fitted into the fitting hole so that the plug is tightly attached to the plug, thereby increasing the sealing performance for sealing between the plug and the side gear. Even when the lubricating oil is removed from the housing, the lubricating oil in the housing can be reliably prevented from flowing out from the axle insertion hole to the outside. Since the plug has a concave cross-section recessed inward in the axial direction of the side gear, the axle can be inserted inside the plug even if the plug is fitted in the fitting hole, so that the axle insertion hole The length of the axle insertion portion to be inserted into the axle need not be shorter than that of the conventional one, and the existing axle can be assembled, and the desired connection strength between the axle and the side gear can be secured.

According to the seal structure of the differential device of the second aspect, the fitting hole portion is provided with the tapered inner surface portion and the step portion, and the plug is provided with the tapered peripheral wall portion. Therefore,
With the end of the peripheral wall portion of the plug almost in contact with the step of the fitting hole, the peripheral wall portion is elastically deformed and firmly fitted inside the tapered inner surface portion of the fitting hole portion to be in close contact therewith. Since the inner diameter of the tapered inner surface of the fitting hole becomes smaller toward the inner side in the axial direction, the plug fitted by fitting the peripheral wall to the inner surface of the tapered inner surface can be surely prevented. In other words, the sealing performance for sealing between the side gear and the axle can be reliably improved, and the plug can be easily and reliably mounted in the fitting hole.

According to the seal structure of the differential device of the third aspect, the taper which guides the peripheral wall portion to be elastically deformed when the plug is mounted at a position axially inner than the tapered inner surface portion of the fitting hole. A guide was provided. Therefore, by pushing the plug into the fitting hole, first, the peripheral wall portion of the plug is guided by the tapered guide portion of the fitting hole so as to be elastically deformed, and then the plug is brought into contact with the pushing step portion. The elastically deformed peripheral wall portion can be fitted inside the tapered inner surface portion. That is, the plug can be easily mounted simply by pushing the plug into the fitting hole.

According to the seal structure of the differential device of the fourth aspect, since the taper angle of the tapered inner surface of the fitting hole is larger than the taper angle of the peripheral wall of the plug, the effect of preventing the plug from coming off is enhanced. Thus, it is possible to further enhance the sealing performance for sealing between the plug and the side gear.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a differential device (a state in which a pair of axles is connected to a pair of side gears) according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a differential device (a state in which a pair of axles is removed from a pair of side gears).

FIG. 3 is an enlarged sectional view of a main part of the differential device.

FIG. 4 is a partial sectional view of a side gear (with a plug removed).

FIG. 5 is a partial sectional view of a side gear (with a plug attached).

FIG. 6 is a cross-sectional view of a differential device (a state in which a pair of axles is connected to a pair of side gears) according to the related art.

FIG. 7 is a cross-sectional view of the differential device (a state in which a pair of axles is removed from a pair of side gears).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Differential device 2 Ring gear 3 Gear case 4 Pinion gear 6 Side gear 6c Axle insertion hole 7 Housing 15 Axle 20 Fitting hole 21 Taper inner surface 22 Step 23, 24 First and second taper guide 30 Plug 31 Peripheral wall

Continuation of the front page (72) Inventor Yu Ogawa 1-1-1 Yidamatsu Iida, Higashihiroshima City, Hiroshima Pref.

Claims (4)

[Claims] 1. A ring gear, a gear case, a pair of pinion gears, a pair of side gears, and a housing for accommodating these members, and a penetrating shape of each side gear whose outer end in the axial direction faces the outside of the housing. In the differential device for a vehicle, in which an axle is inserted into the axle insertion hole and releasably connected to transmit the driving force, an axially inner end portion in the housing of each side gear is recessed outward in the axial direction and In addition to forming a fitting hole communicating with the axle insertion hole, a plug having a concave cross section recessed inward in the axial direction of the side gear is provided, and this plug is fitted inside the fitting hole to allow a space between the plug and the side gear. A seal structure for a differential device, wherein the seal structure is configured to seal and prevent lubricating oil from flowing out of a housing from an axle insertion hole of a side gear. 2. The fitting hole portion has a tapered inner surface portion whose diameter becomes smaller as it goes inward in the axial center direction, and extends from the end of the tapered inner surface portion on the axle insertion hole side toward the shaft center and is larger than the axle insertion hole. 2. A plug having a stepped portion having a diameter, wherein the plug has a tapered peripheral wall portion elastically deformed and fitted inside the tapered inner surface portion in a state of being substantially in contact with the stepped portion. The seal structure of the differential device according to 1. 3. A tapered guide portion is provided at a position axially inner than the tapered inner surface portion of the fitting hole portion so as to guide the peripheral wall portion to elastically deform when the plug is mounted. A seal structure for a differential device according to claim 2. 4. The seal structure for a differential device according to claim 3, wherein a taper angle of a tapered inner surface portion of the fitting hole is larger than a taper angle of a peripheral wall portion of the plug.